Democrats, faced with a daunting set of Senate races in 2012, not only survived but thrived, adding two seats to their majority.

The party will face a difficult map again in 2014, however. Twenty-one of the 35 seats up for election are now held by Democrats. Moreover, most the states that will be casting ballots for the Senate in 2014 are Republican leaning: 7 of the 21 Democratic-held seats are in states carried by the former Republican presidential nominee, Mitt Romney, while just one of the Republican seats is in a state won by President Obama.

Democrats could also suffer from the downside to presidential coattails. Most of the seats up for grabs in 2014 were last contested in 2008, a very strong Democratic year. Without having Mr. Obama on the ballot, and with an electorate that is likely to be older and whiter than in presidential years, some Democrats may find that their 2008 coattails have turned into a midterm headwind instead.

Are the conditions favorable enough to make Republicans odds-on favorites to gain six seats and win the Senate majority? Not quite. Six seats are a lot to gain, and Republicans are at risk of nominating subpar candidates in a number of races. But it would not take all that much to tip the balance toward them.

Our initial snapshot of the 35 Senate races follows below. For each race, I have estimated the rough probability that Democrats and Republicans will hold the seat after the 2014 election. These probabilities are best guesses — they are not determined by a formula — but I have tried to consider the same factors that our statistically driven Senate forecasts will evaluate once we roll them out next year. In particular, I look fairly heavily at the partisan lean of a state, especially as compared with the ideological dispositions of the candidates who might be running there. I consider how easily incumbents won their elections in 2008, and how much experience new candidates have had in running for statewide and federal office before. I look at approval ratings for incumbents (more so than head-to-head polling, which may be unreliable when candidates lack name recognition). And I try to consider what might happen if incumbents who have not yet declared for re-election decide to retire: a Republican retirement in Democratic-leaning Maine would be much more consequential than one in Wyoming.

I make fewer assumptions about what the overall political climate is likely to be in 2014. Midterm years usually favor the party that does not hold the presidency — Republicans, in this case — but generally not to the degree that they favored Republicans in 2010. And if Democrats face some challenges from having become increasingly reliant on voters who do not always turn out in midterm years, Republicans face some branding problems, with party favorability ratings near all-time lows in some polls.

Instead, this is intended as a “bottom-up” view of the Senate picture, with most of the focus on the individual races. We will consider all 35 races at least briefly, but roughly a dozen have a reasonable likelihood of being decisive in determining the Senate majority.

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Safe Democratic

Rhode Island. Senator Jack Reed, a Democrat, is quite popular in Rhode Island, one of the bluest states in the country, and this should be Democrats’ safest Senate race next year.

New Mexico. The disappointing performance of former Representative Heather Wilson, the Republican candidate in New Mexico’s open-seat Senate election last year, suggests that Republicans have little upside against the reasonably popular Democratic incumbent, Tom Udall.

Delaware. There is little approval-ratings data available on Senator Chris Coons, who defeated Christine O’Donnell, a Republican, for his seat in 2010. But Delaware is strongly Democratic and has a history of re-electing incumbents. Barring an unlikely comeback by the former Representative Michael N. Castle, a Republican who would be 75 as of the 2014 election, the seat looks safely Democratic.

Hawaii (special election). The Democratic primary could be competitive in Hawaii after Gov. Neil Abercrombie, facing a choice of several qualified candidates, appointed his lieutenant governor, Brian Schatz, to the seat in December. But there is no indication that the Republicans will be viable in the general election, especially after the former Gov. Linda Lingle, a Republican, lost her Senate race by 25 percentage points in 2012.

Virginia. Senator Mark Warner, a Democrat, is very popular, leaving most of Virginia’s better candidates positioning themselves for the state’s 2013 gubernatorial race instead.

Illinois. Republicans won the Illinois Senate race in 2010, but it required a nearly perfect set of circumstances: the Republicans fielded a strong candidate in Representative Mark Steven Kirk, Democrats had a problematic one in State Treasurer Alexi Giannoulias and it was one of the most Republican-leaning years in decades. Even if longtime Senator Richard J. Durbin, a Democrat, were to retire — and even in a state where the Democratic Party has endured a number of scandals — the odds of everything coming together for Republicans are fairly low.

Likely Democratic

New Jersey. Republicans have had little success in Senate races in New Jersey despite repeatedly facing incumbents like Robert Menendez and Frank R. Lautenberg with middling popularity. Mr. Lautenberg’s recent retirement is therefore unlikely to harm Democrats very much, and instead will avoid what would have been a contentious primary against the mayor of Newark and Democratic front-runner, Cory Booker.

But Mr. Booker, if he has a reputation as a rising star in the Democratic Party, has not run a statewide campaign before and could be mistake prone. There is also the possibility that the Democratic primary could still be competitive if Representative Rush Holt challenges Mr. Booker — or that the ethics accusations surrounding Mr. Menendez somehow reverberate on the race. None of that is enough to make New Jersey a great pickup opportunity for Republicans, but Democrats face some risk of complacency.

Massachusetts (special election in 2013; regular election in 2014). This seat, formerly held by Secretary of State John Kerry, will be contested twice: once in a special election this summer, and then again for a full six-year term in November 2014. In the special election, Republicans will be left to choose from a set of inexperienced candidates after Scott Brown declined to enter the race, leaving Democrats as clear favorites in blue Massachusetts. The question is whether the circumstances that provided Mr. Brown with his upset win in 2010 could somehow be replicated again with a different Republican candidate. At least some of Mr. Brown’s success that year stemmed from the failings of his Democratic opponent, Martha Coakley, and some from a Democratic power grab to change succession laws in the state in 2009. Massachusetts has a large number of independent voters, however, and the 2010 precedent is recent enough that we don’t completely rule out the possibility of a Republican upset. There is also the outside chance that Mr. Brown could choose to run for the full term in 2014, despite passing on the special election.

Michigan. The longtime Democratic incumbent, Carl Levin, has not yet decided whether to run for re-election. If he retires, the race should still be Democratic leaning: Michigan is somewhere between a blue state and a swing state. But Michigan Republicans have a number of competent and moderate representatives whom they could nominate to replace Mr. Levin.

Colorado. The approval ratings of Senator Mark Udall, a Democrat, are middling, but Republicans have nominated a series of weak candidates in statewide races in Colorado and have no clear front-runner to challenge him in 2014.

Oregon. This race obeys a similar dynamic to Colorado: Senator Jeff Merkley, a first-term Democrat, could theoretically be vulnerable based on his approval ratings, but Oregon increasingly leans blue and Republicans have few viable candidates. Because of its mail balloting, Oregon is also a relatively high-turnout state even in midterm years, which could give Mr. Merkley some cushion even if Democrats are unenthusiastic about the midterms elsewhere in the country.

Minnesota. Al Franken is now relatively popular despite having won election by the narrowest of margins (and under disputed circumstances) in 2008; a Star Tribune poll in September put his approval rating at 52 percent among likely voters against 40 percent disapproval. Mr. Franken’s very liberal voting record will nevertheless make him an attractive target for Republicans, but the question is whether they can come up with a candidate who will play effectively to the center. If they choose Representative Michele Bachmann, they will almost certainly squander the opportunity; while a better Republican opponent could move the race into the “leans Democratic” category.

New Hampshire. Jeanne Shaheen, the Democratic incumbent, has strong favorability ratings but her re-election should not be taken for granted. New Hampshire is among the most “elastic” states in the country, meaning that its political climate shifts very rapidly based on national conditions and that the race could be quite competitive if 2014 proves to be a Republican-leaning year.

Leans Democratic

Iowa. The retirement of Senator Tom Harkin, a Democrat, creates a pickup opportunity for Republicans, but one they could waste if they nominate Representative Steve King or another extremely conservative candidate. (In contrast, Democrats are likely to have a competent candidate, most likely Representative Bruce Braley.) The race should be roughly a tossup if Republicans settle on a mainstream candidate like Representative Tom Latham, but the possibility of Mr. King being chosen (one poll already projects him ahead in the Republican primary) leaves Democrats as the modest favorite for now.

Alaska. The next several races feature vulnerable Democratic incumbents in states that were won by Mr. Romney in 2012. All are close calls between the “tossup” and the “lean Democratic” categories. Keep in mind that, as unpopular as Congress is nowadays, incumbents are not usually easy to defeat: only one incumbent senator (Mr. Brown of Massachusetts) lost in the general election in November, and only two (Russell D. Feingold of Wisconsin and Blanche Lincoln of Arkansas, both Democrats) did so in November 2010.

Mark Begich, the Democratic incumbent in Alaska, is one of the more unusual cases: his approval ratings are adequate, despite having a rather liberal voting record in a deeply red state. Part of this may be because Alaska is idiosyncratic, with a large number of independent and libertarian-leaning voters that do not fit neatly into the ideological spectrum. Still, what tips this race slightly into the “lean Democratic” category is the possibility that Joe Miller, a Tea Party favorite, could win the Republican nomination and then face considerable problems next November.

Arkansas. The Democratic incumbent, Senator Mark Pryor, was so formidable in 2008 that he did not draw a Republican challenger. But Ms. Lincoln’s loss in 2010, and the Republican sweep of House races in Arkansas in 2012, should make clear that Mr. Pryor will not get a free pass again. The question is whether Mr. Pryor can successfully distance himself from national Democrats, as Ms. Lincoln failed to do, but as Senator Joe Manchin III, Democrat of West Virginia, did successfully in 2010 and 2012 under broadly similar circumstances. Mr. Pryor’s approval ratings remained decent as of late last year, enough that we consider him a very modest favorite under difficult circumstances, but the race will be a good litmus test of whether the Democratic Party can hold onto seats in the inland South.

Tossup

Montana. Republicans had a disappointing year in Montana in 2012, losing competitive gubernatorial and Senate elections. But the Democratic incumbent up for re-election in 2014, Senator Max Baucus, ensured that this race would be competitive once he inserted himself in the middle of the debate over Mr. Obama’s health care bill in 2009. Mr. Baucus has served Montana in the Senate since 1978, but his approval ratings are now only break even — and in general, we have found that tenure in office is a weak predictor of performance once there is more contemporary evidence about how a state’s voters feel about a candidate.

The wild card is that Mr. Baucus could plausibly be challenged in the Democratic primary by Brian Schweitzer, the state’s former governor, who ended his second term last year with good approval ratings. This is among the relatively rare cases where a successful primary challenge could actually improve a party’s chances in November.

North Carolina. This is a reasonably straightforward race: The approval ratings for Senator Kay Hagan are barely better than break even, which could spell trouble in a state where Democratic turnout drops significantly in midterm years. Still, Republicans face a potentially messy primary, and Ms. Hagan’s condition is not as desperate as that of Democrats like Senator Claire McCaskill who survived under similar circumstances in recent years.

Louisiana. The Democratic incumbent, Mary L. Landrieu, won election very narrowly in 1996, and re-election narrowly in 2002 and 2008. All the while, Louisiana has continued to become redder. While Ms. Landrieu has long sought to cultivate a moderate image, she became entangled in the health care debate in 2009 and 2010 and it is unclear how long she can continue to resist the state’s partisan gravity.

South Dakota. This is one case where Republicans will not have to worry about candidate quality. Mike Rounds, a popular former Republican governor, has already declared for the Senate race. The Democratic incumbent, Senator Tim Johnson, won re-election fairly easily in 2008, but it is hard to know where he stands with South Dakota voters today as the state is rarely polled. Mr. Johnson, who suffered a stroke in 2006, could potentially retire, which would likely leave former Representative Stephanie Herseth Sandlin as the Democratic candidate.

Likely Republican

Maine. Senator Susan Collins, the moderate Republican incumbent, remains extremely popular in Maine. But she is also the only Republican senator facing re-election in a state carried by Mr. Obama, and she is increasingly isolated from other Republicans in the Senate after Mr. Brown and other moderates lost their seats. So the Republicans face a few risks. Ms. Collins has not yet announced whether she will run for re-election and could conceivably retire, as her colleague Olympia J. Snowe did last year. She could lose in a primary challenge. She could even switch parties, which would count as a Democratic pickup for our purposes. Estimating the exact likelihood of these outcomes is challenging, but the attrition rate for moderate Republicans has been high in recent cycles, especially in New England. Until Ms. Collins’s plans become clearer, we think this race belongs somewhere on the threshold between likely and leaning Republican.

West Virginia. West Virginia is the Senate seat most likely to switch sides in 2014, as Republicans have a potentially strong candidate in Representative Shelley Moore Capito to replace the retiring Democratic incumbent, John D. Rockefeller IV. Conservative groups might try to nominate a more conservative candidate to challenge Ms. Capito in the Republican primary, but it is unclear if such efforts have much likelihood of succeeding in a state that remains fairly moderate on economic policy, even as it is strongly culturally conservative.

Some other forecasters are characterizing West Virginia a “tossup,” but we do not see much justification for that, even accounting for the state’s idiosyncrasies. Certainly, the Democratic Party remains in better shape in West Virginia than you might expect based on the state’s recent presidential voting, as Democrats control the state’s Legislature and governorship. But federal offices are a different matter, and Democrats are unlikely to come up with a candidate voters can trust as much as Mr. Manchin to differentiate himself from Mr. Obama. Early polling by a Republican-leaning firm puts Ms. Capito well ahead of one of the stronger potential Democratic nominees, Representative Nick J. Rahall II, while other viable Democrats are avoiding the race.

Georgia. Republican voters are likely to have a wide field to pick from to replace their retiring incumbent Saxby Chambliss, probably including several very conservative representatives who could prove to be high-risk candidates in the general election. But it is unclear whether Democrats are positioned well enough in Georgia to take advantage of the opportunity. The state has been modestly competitive in the past two presidential elections, but that is partly because of high turnout among African-American and college-age voters for Mr. Obama, groups that are less likely to turn out in midterm years. Polling suggests that moderate Democrats like Representative John Barrow or former Gov. Roy Barnes could be tough opponents, but neither has declared for the race, nor have Democrats succeeded in recruiting any major candidates so far.

Kentucky. The Republican minority leader, Senator Mitch McConnell, is not terribly popular in Kentucky, although the polls differ on exactly how acute his problems are. A December poll by the Democratic-leaning firm Public Policy Polling put his approval rating at just 37 percent, while a Louisville Courier-Journal poll in September had him with a 51 percent approval rating.

Some Democrats are intrigued by the possibility that the actress Ashley Judd could challenge Mr. McConnell, and some Republicans seem nervous about those prospects (Mr. McConnell, who is already running Web-based ads against Ms. Judd, is apparently among them). On the basis of the fundamentals, however, it is hard to see her as all that much of a threat. Ms. Judd has never run for office, something that we have found often produces underachieving candidates, and she would undoubtedly be associated with a fairly liberal set of policy views in a conservative state. If Mr. McConnell’s approval ratings are truly as low as 37 percent then a lot of things could happen, including a primary challenge or other Democrats entering the race, but otherwise this looks like an uphill battle for Democrats.

Safe Republican

South Carolina (special election) and (regular election). Two Republican-held seats will be up for election in South Carolina in 2014, one held by Lindsey Graham and the other by Tim Scott, who was appointed to his seat in January and who will face a special election. Mr. Graham’s approval ratings are average, and appointed incumbents like Mr. Scott often face competitive races; either or both could also potentially have to endure a primary battle. However, Democrats have long had tremendous difficulty nominating high-quality candidates in South Carolina, and their best prospects could instead be attracted to the governors’ race, where the Republican incumbent, Nikki R. Haley, is vulnerable. Instead, Mr. Scott is likely to become the first African-American to be directly elected to the Senate from a Southern state.

Texas. John Cornyn’s approval ratings are mediocre, but the same was true in 2008 and Mr. Cornyn had little trouble defeating the Democratic nominee, Rick Noriega. There is not yet much evidence that the Texas’s increasingly Hispanic population has translated into Democratic gains in statewide races. Instead, Texas has behaved a bit like the Republican counterpart to New Jersey, a state where theoretically vulnerable incumbents manage to get by.

Nebraska. Republican Senator Mike Johanns is retiring after just one term, but Democrats are unlikely to make it a competitive race. Their candidate in 2012, Bob Kerrey, the former Nebraska senator and governor, lost badly to Deb Fischer, his inexperienced Republican opponent, suggesting that Nebraska is moving from being a state that sometimes crosses party lines in Congressional elections into a reliably Republican one. Republicans could make Democratic prospects even more remote if Gov. Dave Heineman runs for the seat.

Tennessee. Lamar Alexander, the popular Republican incumbent, has been trying to pre-empt a primary challenge and may have deterred conservative groups from backing one. Even if such a challenge were to succeed, Democrats have few top candidates in Tennessee; former Gov. Phil Bredesen, a Democrat, has said that he is not interested in the race.

Lightning round. In Mississippi, Thad Cochran, the Republican incumbent, will be 76 in 2014 and could retire, but Democrats have not won a Senate election there since 1982. In Kansas, the Republican winning streak is even longer: Democrats last won a Senate race in 1932, and Pat Roberts, a Republican, should win re-election. Whatever remote Democratic prospects there are in Oklahoma probably depend on former Gov. Brad Henry running after a Republican retirement, but the Republican incumbent, James Inhofe, is running for another term. A potential retirement by Michael Enzi, the Republican incumbent, could prompt Elizabeth Cheney, the daughter of former Vice President Dick Cheney, to run in Wyoming, bringing marginally more attention to the race but not making it significantly more competitive in one of the nation’s reddest states. There are no problems on the horizon for Jeff Sessions, the Republican incumbent in Alabama, or for Jim Risch, the first-term Republican incumbent in Idaho.

The Big Picture

Summing up the possibilities across all 35 Senate races yields a net gain of four to five seats for Republicans, just short of the six they would need to win back the majority.

However, the margin of error on the calculation is very high at this early stage. Keep in mind that in each of the last four cycles, one party (Democrats in 2006, 2008 and 2012; Republicans in 2010) won the vast majority of the competitive races. If Republicans swept all the “lean” and “tossup” races, they would gain a net of eight seats from Democrats, giving them a 53-to-47 majority in the 114th Congress. If Democrats swept instead, they would lose just one seat and would hold a 54-to-46 majority. Considering the uncertainty in the landscape, estimates from betting markets that Democrats have about a 63 percent chance of holding their majority appear to be roughly reasonable.

One last factor to consider is that as difficult as the Democratic Senate map looks in 2014, Republicans could face an equally challenging one in 2016. In that year, seven Republican-held seats will be up in states won by Mr. Obama in 2012, while no Democrats will face re-election in states won by Mr. Romney.

Thus, as ridiculous as it might seem to look so far ahead, the most important reverberations from the 2014 Senate races might not be felt until 2016 and beyond. Republicans will need to make considerable gains next year to open up the possibility of a Republican-controlled Congress after 2016. If Democrats hold their ground, conversely, it would provide for the outside possibility of their holding a filibuster-proof majority after 2016.