With the field of Senate contenders coming into focus, Democrats see 2016 as a prime opportunity to regain a majority in the upper chamber.

The Democrats are likely to benefit from coattails in several battleground states that tilt blue in presidential election years.

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They believe landing New Hampshire Gov. Maggie Hassan this week to run against Republican Sen. Kelly Ayotte Kelly Ann AyotteBottom line Bottom line Bottom Line MORE is the capstone on a near-perfect recruitment run.

But Republicans will not go quietly.

The GOP’s stable of experienced incumbents will be well-funded and benefit from heavy spending from outside conservative groups.

Republicans scoff at the Democrats’ recruitment haul, calling candidates in states such as Ohio and Wisconsin “retreads.” They’re also pulling for bloody primary fights among Democrats in Florida, Illinois and Pennsylvania.

The GOP is defending 24 seats this cycle, compared to only 10 for Democrats. Right now, much of the focus is on nine states. President Obama carried all nine in 2008 and seven of them in 2012.

If Democrats can net five seats in total between those contests, they’ll reclaim a majority in the Senate.

Here’s a look at the state of the Senate races based on interviews with insiders from both parties.

ENDANGERED REPUBLICANS: Illinois and Wisconsin

Two one-term Republican senators from neighboring states in the Midwest, Sen. Mark Kirk Mark Steven KirkLiberal veterans group urges Biden to name Duckworth VP On the Trail: Senate GOP hopefuls tie themselves to Trump Biden campaign releases video to explain 'what really happened in Ukraine' MORE in Illinois and Sen. Ron Johnson Ronald (Ron) Harold Johnson CIA found Putin 'probably directing' campaign against Biden: report This week: Supreme Court fight over Ginsburg's seat upends Congress's agenda GOP set to release controversial Biden report MORE in Wisconsin, face tough reelection prospects.

Kirk is easily the most vulnerable incumbent of the cycle.

In deep-blue Illinois, Kirk will have to run about 10 points ahead of the Republican presidential candidate in the state, estimates Geoffrey Skelley of the University of Virginia’s Center for Politics.

That would mean running a near-perfect campaign, and few believe that Kirk, with his propensity for grabbing controversial headlines, is a candidate capable of pulling that off.

Republicans argue that Kirk has cut a moderate profile in the Senate and note that he has outperformed in a swing district in the House.

They also point to the Democratic primary in the state that has pitted the party’s establishment, which is backing Rep. Tammy Duckworth, against many in the African-American community, who support former Chicago Urban League President Andrea Zopp.

But whoever emerges from that contest will be the heavy favorite to defeat Kirk and return President Obama’s old Senate seat to Democrats.

In Wisconsin, Johnson’s prospects appear better, but not by much. Polling shows former Democratic Sen. Russ Feingold with a healthy lead early on. Johnson defeated Feingold in 2010, but that was on much more favorable political terrain for Republicans.

Johnson will have to upset a pattern of Democratic Senate contenders running even with the presidential candidate in a state that hasn’t gone for the Republican presidential nominee in more than 30 years.

REPUBLICAN FIREWALL: Ohio, New Hampshire and Pennsylvania

For Republicans to maintain the Senate, they’ll need to win at least one of these three swing states that have gone for the Democratic presidential nominees in recent elections.

Republicans believe that in Sens. Rob Portman Robert (Rob) Jones PortmanMcConnell locks down key GOP votes in Supreme Court fight Romney undecided on authorizing subpoenas for GOP Obama-era probes Congress needs to prioritize government digital service delivery MORE (R-Ohio), Kelly Ayotte (R-N.H.) and Pat Toomey (R-Pa.), they’re trotting out strong, well-funded candidates capable of delivering, even if their states go blue in the presidential race.

In Ohio, Portman has so far run the strongest campaign of any candidate on either side. Portman has rolled out support from nearly the entirety of the GOP establishment in the Buckeye State; has been burying his Democratic challenger, former Ohio Gov. Ted Strickland, with an early ad barrage; and sits on a whopping $11 million in campaign cash.

Democrats acknowledge they’re going to be outspent in the race but need Strickland to at least be competitive on the money front. Strickland’s third-quarter Federal Election Commission filing will say a lot about his strength as a challenger.

In New Hampshire, Ayotte has a tough draw in Hassan, a top-tier candidate who may be the biggest get for Democrats this cycle. Insiders on both sides say that with Hassan running, the race is a true toss-up.

The two experienced, well-known and admired female candidates pitted against one another in a high-stakes contest in New Hampshire — where the presidential spotlight will be bright — will make this one of the most competitive races of the cycle.

In Pennsylvania, Toomey is in the most precarious position of the three.

The Keystone State is solid blue in presidential election years. However, Toomey stays out of the Endangered category by virtue of his substantial early fundraising advantage, his deep ties to conservative groups that will work tirelessly on his behalf and the fact that Pennsylvania has a history of ticket splitting.

In addition, a strong Democratic challenger has yet to emerge.

Establishment Democrats appear to be gravitating toward Katie McGinty, the former chief of staff to Pennsylvania Gov. Tom Wolf. Republicans gleefully point out that McGinty finished last in the state’s gubernatorial primary in 2014.

But suburban Pittsburgh Mayor John Fetterman is running a serious campaign, and former Rep. Joe Sestak, though despised by many Democrats, is still hanging around. Sestak lost to Toomey by only two points in 2010, a wave election year for Republicans.

Depending on the Democratic winner, Toomey could be in big trouble.

WILDCARDS: Florida and Nevada

Candidates on both sides have much to prove in these two purple states.

On the Democratic side in Florida, Reps. Patrick Murphy and Alan Grayson Alan Mark GraysonFlorida's Darren Soto fends off Dem challenge from Alan Grayson Live results: Arizona and Florida hold primaries The Hill's Morning Report: Frustration mounts as Republicans blow up tax message MORE are intent on embarrassing and destroying one another.

Establishment Democrats are firmly behind Murphy, who they believe will be a top-tier general-election candidate. But they worry that Grayson, a liberal firebrand, could do real damage to Murphy.

Grayson’s third-quarter fundraising haul will be closely watched to see if he’s able to deliver on his boasts of having one of the largest national grassroots donor bases in Congress. Of course, the independently wealthy Grayson could self-fund his campaign.

Florida Republicans face a drawn-out primary of their own, with four candidates in the race and potentially more to come.

Rep. Ron DeSantis, an Iraq War veteran, has the backing of several fiscally conservative groups. Carlos López-Cantera, Florida’s first Hispanic lieutenant governor, is in. So is Rep. David Jolly, a first-term House member who surprised political watchers by edging Democrat Alex Sink in a special election last year.

Primaries are not an issue in Nevada, where Democrats and Republicans are both bullish on their prospects.

Both parties are running candidates who look excellent on paper but who will be running alone statewide for the first time in the Silver State, which went for Obama in 2008 and in 2012.

Democrats are running a Hispanic woman, former Attorney General Catherine Cortez Masto, who is Sen. Harry Reid Harry Mason ReidThe Supreme Court vacancy — yet another congressional food fight Trump seeks to turn around campaign with Supreme Court fight On The Trail: Battle over Ginsburg replacement threatens to break Senate MORE’s handpicked successor. On the Republican side, Rep. Joe Heck is an Iraq War veteran who has run strong campaigns before in his swing district.

NO-SHOWS: Colorado and North Carolina

Colorado Sen. Michael Bennet Michael Farrand BennetOVERNIGHT ENERGY: House Democrats tee up vote on climate-focused energy bill next week | EPA reappoints controversial leader to air quality advisory committee | Coronavirus creates delay in Pentagon research for alternative to 'forever chemicals' Senate Democrats demand White House fire controversial head of public lands agency Next crisis, keep people working and give them raises MORE should be the most vulnerable Democrat running for reelection, but Republicans can’t find anyone to run against him.

The list of Republicans who have passed on the race is long, leaving the party to rely on a handful of untested or obscure candidates.

Republicans point to then-Rep. Cory Gardner Cory Scott GardnerOvernight Health Care: US coronavirus deaths hit 200,000 | Ginsburg's death puts future of ObamaCare at risk | Federal panel delays vote on initial COVID-19 vaccine distribution The Hill's Campaign Report: GOP set to ask SCOTUS to limit mail-in voting Senate GOP sees early Supreme Court vote as political booster shot MORE’s late entrance in the race in 2014 as evidence that a dark horse could still emerge, but Bennet’s campaign is in full swing, and he’s racked up several strong fundraising quarters already.

That same scenario is playing out for Democrats looking to take out Republican Sen. Richard Burr Richard Mauze BurrRep. Mark Walker says he's been contacted about Liberty University vacancy Overnight Defense: Trump rejects major cut to military health care | Senate report says Trump campaign's Russia contacts posed 'grave' threat Senate report describes closer ties between 2016 Trump campaign, Russia MORE in North Carolina.

Former Sen. Kay Hagan Kay Ruthven HaganThe Hill's Campaign Report: Democratic Unity Taskforce unveils party platform recommendations Democrats awash with cash in battle for Senate The Hill's Campaign Report: Trump's job approval erodes among groups that powered his 2016 victory MORE will not be making a comeback, and Transportation Secretary Anthony Foxx Anthony Renard FoxxBig Dem names show little interest in Senate Lyft sues New York over new driver minimum pay law Lyft confidentially files for IPO MORE also has declined to run, leaving Democrats to plumb the depths of their second tier.

Democrats similarly argue that it’s too early to panic, pointing to former Sen. Hagan’s late entrance into the 2008 Senate race, when she defeated former Republican Sen. Elizabeth Dole.