This weekend’s announcement by the former governor of Montana, Brian Schweitzer, that he would not seek that state’s Democratic nomination for Senate represents the latest in a series of favorable developments for Republicans as they seek control of the chamber.

The G.O.P.’s task will not be easy: the party holds 46 seats in the Senate, and the number will very probably be cut to 45 after a special election in New Jersey later this year. That means that they would need to win a net of six contests from Democrats in order to control 51 seats and overcome Vice President Joseph R. Biden Jr.’s tiebreaking vote. Two years ago at this time, Republicans faced what seemed to be a promising environment and could have won the Senate by gaining a net of three seats from Democrats and winning the presidency. Instead, Mitt Romney lost to President Obama, and the G.O.P. lost a net of two Senate seats.

But Montana along with West Virginia and South Dakota — two other red states where an incumbent Democrat has retired and where the Democrats have not identified a strong candidate to replace them – gives Republicans a running start. Republicans could then win three more seats from among red states like Louisiana and Arkansas, where vulnerable Democratic incumbents are on the ballot, or they could take aim at two purple states, Iowa and Michigan, where Democrats have retired. More opportunities could also come into play if the national environment becomes more favorable to Republicans (such as because of a further slide in Mr. Obama’s approval ratings). Meanwhile, while Kentucky and Georgia are possibly vulnerable, Republicans have few seats of their own to defend; unlike in 2012, they can focus almost entirely on playing offense.

A race-by-race analysis of the Senate, in fact, suggests that Republicans might now be close to even-money to win control of the chamber after next year’s elections. Our best guess, after assigning probabilities of the likelihood of a G.O.P. pickup in each state, is that Republicans will end up with somewhere between 50 and 51 Senate seats after 2014, putting them right on the threshold of a majority.

The chart below reflects our current overview of the Senate landscape, including the probability estimates. (The estimates are not based strictly on a formula but instead are best guesses, accounting for the partisan lean of each state, the quality of the prospective candidates, and approval-rating or polling data to the extent that it might be informative.) When we last conducted this exercise in February, our projection was for Republicans to win between 49 and 50 Senate seats, meaning that their standing has improved by about one seat since then.

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What follows is a series of comments on the most critical races, or those where there has been significant news since February. The races are listed in order of the contests that look strongest for Democrats to those where Republican chances are strongest.

Massachusetts. Although the Democrats’ position has become more vulnerable over all, one state where they have less reason to worry is Massachusetts, where Representative Edward J. Markey defeated the Republican Gabriel Gomez by 10 percentage points in a special election in June. The special election reflected reasonably favorable circumstances for Republicans relative to the usual (very difficult) ones in Massachusetts – an extremely low turnout, and a Democratic candidate who had survived a tough primary. But Republicans did not come particularly close to winning, and November’s environment will be no easier for the G.O.P. as Mr. Markey will have gained experience as the incumbent and turnout will probably be high because of a governor’s election.

New Jersey. This seat is currently held by a Republican — Jeffrey Chiesa, the former state attorney general who was appointed to the seat by Gov. Chris Christie after the death of the Democratic incumbent Frank R. Lautenberg. However, it is likely to revert to Democratic control later this year, as the timing of the October special election (and current polling) suggests that Republicans will not nominate a viable opponent against the likely Democratic nominee, Mayor Cory Booker of Newark. Although Republicans could regroup to nominate a stronger candidate in 2014, when the special election winner will stand for re-election, a candidate like Mr. Booker will have gained more experience by that time, as well as the financial advantage that comes with being an incumbent.

Colorado, Minnesota, New Hampshire and Oregon. These states fall into the “likely Democratic” category, where Democratic odds of holding the seats vary between about 75 percent and 85 percent, in our view. They consist of two purple states, Colorado and New Hampshire, where a reasonably popular Democratic incumbent is running, and two others, Minnesota and Oregon, where the Democratic incumbent’s numbers are more mixed but which are somewhat blue-leaning. In each state, Republicans have had trouble recruiting top-tier challengers. To make the races competitive, Republicans would probably need at least two things to go well: first, a stronger G.O.P. challenger; and second, either a shift in the national political environment toward Republicans or some other circumstance (such as a controversial vote or an ethics problem) that made the incumbents more vulnerable. At this early stage of the election cycle, either is possible in each state, and if it were to occur in one or more of these states the Republican path to a Senate majority would be more robust. But these races will gradually drift toward Democrats absent such changes in the landscape.

Michigan and Iowa. These are states where Democratic incumbents have retired, but where Democrats retain an edge in candidate strength. The G.O.P. avoided their worst-case scenario in Iowa after Representative Steve King declined to run for the seat there, as Mr. King would likely have been far too conservative to win a statewide race. But Republicans have nevertheless had trouble identifying strong candidates to run in Iowa, whereas Democratic Representative Bruce Braley could win his primary easily and conserve money for the general election. In Michigan, the G.O.P.’s list of potential candidates is slightly better, but their strongest potential candidate, Representative Mike Rogers, has decided not to run — and Michigan has become somewhat blue-leaning in races for federal office.

Alaska. Alaska is more idiosyncratic than its reliably Republican results in presidential elections might imply, and the Democratic incumbent, Mark Begich, is faring reasonably well in head-to-head polls. A good Republican nominee could nevertheless turn the race into a true tossup — but the G.O.P. could also nominate Joe Miller or former Gov. Sarah Palin, both of whom are now quite unpopular among voters statewide. The Republican primary outcomes in Alaska, Iowa and Michigan will thus go a long way toward determining whether the G.O.P.’s path to a Senate majority is a narrow or wider one.

North Carolina. North Carolina is the closest thing to the tipping point state in the Senate battle. If Republicans avoid losing any of their own seats (other than New Jersey), and win the races that favor them in Montana, West Virginia and South Dakota and the tossup races in Louisiana and Arkansas, then North Carolina would represent their best option to go from a 50-50 tie to a 51-49 Senate majority. Republicans have also not yet identified a top-tier challenger in North Carolina. But the approval ratings of the Democratic incumbent, Kay Hagan, are no better than break-even, which means that a merely decent Republican nominee could make the race very competitive. Although North Carolina is increasingly purple in presidential election years, the coalition of African-Americans and college-aged voters that Democrats depend upon to win races in the state is less likely to turn out for midterm elections.

Arkansas. We had this race rated as “lean Democratic” in February on the basis of reasonably strong approval ratings for Mark Pryor, the Democratic incumbent, despite an otherwise difficult environment in the state. However, a poll that came out since then shows middling numbers for Mr. Pryor, with 41.5 percent of voters approving of his job performance against 35 percent disapproving. Mr. Pryor is a savvier politician than the former Democratic senator, Blanche Lincoln, and he is unlikely to face a serious primary challenge, as Ms. Lincoln did in 2010. But Ms. Lincoln lost her race for re-election by more than 20 percentage points, so Mr. Pryor will have to avoid several of her mistakes to hold on to his seat.

Louisiana. This race also rates as a true tossup, as it did in February. The most likely Republican nominee, Representative Bill Cassidy, has an extremely conservative voting record, while the Democratic incumbent, Mary Landrieu, has a moderate one that could partially offset the state’s strong Republican lean. But Ms. Landrieu will need to maintain her distance from President Obama while at the same time getting a strong turnout in New Orleans, where her bother Mitch Landrieu is mayor — a difficult balance.

Montana. The decision of Mr. Schweitzer not to enter the race leaves wide-open primary fields in both parties. However, we have Republicans as roughly three-to-one favorites based on the state’s overall partisan lean. Although Democrats have had somewhat more success in statewide races in Montana than in contests for the presidency — their incumbent senator, Jon Tester, survived a tough re-election race last year — Republicans now have the best-qualified potential candidate in Marc Racicot, the former governor.

Kentucky. Senator Mitch McConnell, the Republican minority leader, has only break-even approval ratings, and Democrats got one of their better potential recruits in Alison Lundergan Grimes, the Kentucky secretary of state. Although some early polls show a relatively close race, the fundamentals favor Mr. McConnell as Kentucky has become very red-leaning and as he is likely to have a strong fund-raising edge. One factor helping Ms. Grimes is the lack of other credible pickup opportunities for Democrats, which could mean that more Democratic money will be directed toward her race. Still, she will have to run a pitch-perfect campaign to win in such a conservative state in a midterm year.

Georgia. This is something of an overlooked pickup opportunity for Democrats. While Democrats have yet to identify a strong candidate, a number of the Republican ones are extremely conservative, Tea Party-backed candidates who could be vulnerable in the suburban counties around Atlanta and Augusta that make up the swing regions of Georgia. Nor is Georgia as conservative as Kentucky — although as in North Carolina, Democrats depend on African-Americans and young voters to make the state more purple, groups that may not turn out for the midterms.

Maine. Senator Susan Collins, the popular and moderate Republican incumbent, has clarified her plans to run for re-election, which reduces the risk to Republicans of losing a seat. We give Democrats a small, residual chance of a pickup here — about 10 percent — because if Ms. Collins loses in a primary, or has a change of plans, the race would instantly go to favoring Democrats. But she will be an overwhelming favorite if she stands on next November’s ballot.

South Dakota and West Virginia. These are red states where Democratic incumbents are retiring — and where some of the more viable Democratic alternatives, like former Representative Stephanie Herseth Sandlin of South Dakota — have declined to run. Meanwhile, Republicans have very strong recruits in both states in the form of Representative Shelley Moore Capito of West Virginia; and Mike Rounds, the former South Dakota governor. Democrats have had more success in Congressional races in both of these states than in presidential contests, and they might stand a decent chance to retain these seats with a candidate-quality edge. But the combination of a conservative electorate, a very strong G.O.P. nominee and an underqualified Democratic one will make their chances very slim. In addition, our research does not find much evidence for a carry-over effect or afterglow from incumbency once the incumbent retires; now that Senators John D. Rockefeller IV of West Virginia and Tim Johnson of South Dakota are leaving politics, they won’t provide much in the way of electoral benefit to Democrats. Add all these factors up, and we don’t see much justification for describing these races as tossups or as merely “leaning” toward Republicans, as some other forecasters have them: they are pretty safe bets to be Republican pickups.

Nebraska. Republicans suffered a recruiting blow in Nebraska when the popular Gov. Dave Heineman said he would not run in the Senate race. But Mr. Heineman’s decision improves Democrats’ chances only to slim from none. In 2012, Democrats nominated an extremely qualified candidate in Bob Kerrey, the former senator and governor, while Republicans had a middling one in Deb Fischer, the state senator — but Ms. Fischer won overwhelmingly.

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Let me close with our usual reminder: the fact that the battle for Senate control appears to be very close right now does not guarantee that it will end up that way. Although Senate races behave more idiosyncratically than some other types of contests — local factors and candidate quality play an important role — one party has won the vast majority of tossup races in each of the past four election cycles.

In a strong Republican year, the G.O.P. could win all of the tossup and “lean Democratic” seats and pick up one of the “likely Democratic” seats like New Hampshire, which would give them a net gain of nine seats and leave them with a 55-45 majority in the chamber. In a strong Democratic year, the party could lose only West Virginia and South Dakota – and pick up New Jersey and one of Kentucky and Georgia – and hold their current 54-46 edge. It is therefore important to watch macro-level indicators – especially Mr. Obama’s approval ratings, the generic Congressional ballot and major economic measures – in addition to following the recruitment and polling in individual states.

It is equally important to look for early indications of whether G.O.P. primary voters will be more tolerant of moderate and “main street” Republicans than they were in 2010 and 2012. A strong set of Republican nominees could give the party as many as a dozen credible opportunities to pick up the seats they need – whereas a weaker series of candidates could require them to win almost all of the races that remained competitive after the primaries.