Senate Republicans will have to work hard to retain their recently won majority as they face a tough 2016 electoral map.

They have 24 seats up compared to Democrats' 10, including seven in states President Obama carried twice. Democrats won't have any red-state senators facing reelection and could be buoyed by a favorable presidential-year electorate.

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Republicans do have some margin for error after their sweeping 2014 win netted them nine seats for a 54-seat Senate majority. Democrats won't have it easy either, with Senate Leader Harry ReidHarry ReidDems face identity crisis Heller under siege, even before healthcare Charles Koch thanks Harry Reid for helping his book sales MORE (Nev.) facing a strong challenge.

But Democrats are cautiously optimistic they can win back control just two years after losing it — and Republicans admit that they have a fight on their hands.

“It'll be tough but it's definitely not impossible. We only need four seats if we win the White House and we start off with four very vulnerable Republicans,” said one national Democratic strategist.

“There are just some hardcore blue presidential states,” said a national Republican strategist.

Here are 10 senators in danger of losing in 2016.

1. Sen. Mark KirkMark KirkGOP senator defends funding Planned Parenthood Why Qatar Is a problem for Washington Taking the easy layup: Why brain cancer patients depend on it MORE (R-Ill.)

Kirk narrowly defeated a scandal-plagued Democratic candidate by less than 2 points in the 2010 Republican wave election. In the 2016 presidential election year, he will likely face a much more Democratic electorate.

The senator has worked hard to bolster his bipartisan credentials, backing immigration reform, gun control and environmental protections. He was also the second Republican senator to embrace gay marriage.

His hard-fought battle to return from a debilitating stroke could also earn him sympathy from voters.

Kirk told The Hill in November that he’s running “come hell or high water,” seeking to dispel rumors he might retire.

But in deep-blue Illinois even the strongest Republicans face uphill battles. Democrats are eying Illinois Attorney General Lisa Madigan (D) and Reps. Tammy Duckworth (Ill.), Cheri BustosCheri BustosLawmakers send well-wishes to Scalise on Twitter Dem lawmaker: Time for new generation of leadership Members jam with Wynonna Judd, Keith Urban at Grammys on the Hill MORE (Ill.) and Bill FosterBill FosterHouse GOP’s new challengers: Scientists mulling campaigns Dems crowd primaries to challenge GOP reps Lawmakers talk climate for Earth Day, Science March MORE (Ill.) as potential challengers.

 

2. Sen. Ron JohnsonRon JohnsonHealthcare wish lists: What moderates, conservatives want Overnight Healthcare: GOP infighting erupts over bill | How Republican governors could bring down ObamaCare repeal | Schumer asks Trump to meet with Dems GOP infighting erupts over healthcare bill MORE (R-Wis.)

Polls show Johnson isn’t that well-known or well-liked in Democratic-leaning Wisconsin, where he won by 5 points in the 2010 GOP wave.

He may face a rematch against Sen. Russ Feingold (D-Wis.) in a high-turnout election year. Adding to his worries: No GOP presidential nominee has carried Wisconsin in more than three decades.

Johnson has done little to build bipartisan credentials in the Democratic-leaning state, and Feingold is giving serious consideration to another run.

The big question is whether the former senator has learned his lesson after refusing outside money in 2010, letting Johnson outspend him by a wide margin, and declining to air any attack ads. If Feingold is the nominee and runs a smarter campaign this time around, Johnson could face an uphill battle to hold his seat.

 

3. Sen. Harry Reid (D-Nev.)

Polls show Reid’s approval numbers are underwater in his home state. Strategists in both parties say he’d be the underdog if popular Nevada Gov. Brian Sandoval (R), who just won reelection with more than 70 percent of the vote, decides to run against him.

Even if Sandoval doesn’t run, Democrats admit Reid may have a tough fight on his hands if Republicans can find a strong candidate.

But Nevada has been trending Democratic due to its fast-growing Hispanic population, and the party tends to do much better there in presidential years. Reid also won by a surprisingly comfortable margin in 2010 against a deeply flawed Republican candidate, and has proven to be a scrappy campaigner.

His numbers, though, just aren’t that good in Nevada — and the soon-to-be Senate minority leader is likely to face a tough fight.

 

4. Sen. Pat Toomey (R-Pa.)

Toomey faces a potential rematch against former Rep. Joe Sestak (D-Pa.), who he defeated by a narrow margin in 2010.

The senator has worked hard to shed his image as an unyielding Tea Party candidate, working with Democrats on fiscal issues and taking the lead in crafting gun-control legislation with Sen. Joe ManchinJoe ManchinFood Network star honors veterans with dessert feast Dems face identity crisis This week: Senate races toward ObamaCare repeal vote MORE (D-W.Va.).

Democrats admit he’ll be a tough out — but they believe Pennsylvania’s blue lean in presidential years means Toomey's in trouble.

 

5. Sen. Kelly AyotteKelly AyotteOPINION: Democracy will send ISIS to the same grave as communism Kelly Ayotte joins defense contractor's board of directors Week ahead: Comey firing dominates Washington MORE (R-N.H.)

Ayotte hails from more of a swing state than Toomey, Kirk and Johnson, and Republicans believe she’s done well to shore herself up in New Hampshire.

But Democrats think she’s beatable if they find the right candidate — and are hoping they can convince Gov. Maggie Hassan (D-N.H.) to run.

 

6. Sen. Richard BurrRichard BurrOvernight Cybersecurity: New questions about 'ransomware' attack | Tensions between NSA chief, Trump over Russia | Senate panel asks states to publicize election hacks Trump trolling of Comey — Not presidential Senate panel to get Comey memos: report MORE (R-N.C.)

Polls show Burr isn’t that well-known in North Carolina, a tremendously difficult state to build name recognition because of its 15 distinct media markets. He also has little money in the bank for his reelection bid.

Burr has worked hard to establish himself as an even-tempered lawmaker and while North Carolina has moved Democratic in recent years it still leans slightly Republican overall.

Democrats are hoping they can find a top-tier candidate to challenge him — potentially outgoing Sen. Kay HaganKay HaganLinking repatriation to job creation Former Sen. Kay Hagan in ICU after being rushed to hospital GOP senator floats retiring over gridlock MORE (D-N.C.), who lost a close race last month. They concede that the quality of their candidate and the national political climate will likely determine whether or not they can compete in North Carolina.

 

7. Sen. Michael BennetMichael BennetDems step up attacks on GOP ObamaCare bill Trump welcomes Gorsuch on first Supreme Court visit Why higher education is in need of regulatory relief MORE (D-Colo.)

Bennet narrowly won in 2010 against a gaffe-prone Republican and is coming off a rough cycle as chairman of the Democratic Senatorial Campaign Committee. He also just lost his home-state colleague, outgoing Sen. Mark UdallMark UdallDemocratic primary could upend bid for Colorado seat Picking 2018 candidates pits McConnell vs. GOP groups Gorsuch's critics, running out of arguments, falsely scream 'sexist' MORE (D-Colo.), defeated by Sen.-elect Cory GardnerCory GardnerActivists spend 24 hours in GOP senator's office to protest ObamaCare repeal New Alexandra Pelosi documentary brings together GOP, Dem members GOP ObamaCare fight faces do-or-die procedural vote MORE (R-Colo.).

Colorado has trended Democratic but remains a pure toss-up state — as Gardner’s win shows. The big question is whether Republicans can find another candidate of Gardner’s quality, as strategists admit they have a relatively thin bench in the state.

Rep. Mike Coffman (R-Colo.) could be a top recruit that could put the seat in play.

 

8. Sen. Marco RubioMarco RubioCapitol Police arrest 40 during healthcare protests New Alexandra Pelosi documentary brings together GOP, Dem members Senators urge Trump to do right thing with arms sales to Taiwan MORE (R-Fla.)

Rubio has said he won’t run for both reelection and the presidency, and the swing-state seat would become a tossup if he decides to focus on a White House bid.

Democrats contend they have an outside shot at Rubio even if he runs for reelection, and tout potential candidates including Rep. Patrick Murphy (D-Fla.) and Rep.-elect Gwen Graham (D-Fla.). Both are fresh-faced members who won in Republican-leaning districts. Democratic National Committee Chairwoman Debbie Wasserman Schultz (D-Fla.) is another possibility.

  

9. Sen. Rob PortmanRob PortmanHealthcare wish lists: What moderates, conservatives want GOP infighting erupts over healthcare bill Our national parks need updates; Congress can help MORE (R-Ohio)

Portman recently declared he’ll run for reelection instead of making a White House bid, and strategists in both parties say he’ll be tough to beat. Portman is a fundraising powerhouse and has done little to stir controversy while in the Senate.

But Democrats are hopeful they can compete in swing-state Ohio if they land a top recruit — and are mentioning former Ohio Gov. Ted Strickland (D) as the type of candidate who could give him a tough fight.

 

10. Sen. Lisa MurkowskiLisa MurkowskiBudowsky: Rising up vs. TrumpCare Trump: Senate GOP 'very close' to agreement on health bill EPA head faces skeptical senators on budget cuts MORE (R-Alaska)

Murkowski is by far Alaska’s most popular politician, and has been preparing for reelection ever since her surprise 2010 primary loss and subsequent write-in general election victory.

But her old foe, 2010 Senate nominee Joe Miller (R), may give her another challenge, and her support for abortion rights, gay marriage and centrist fiscal and energy positions gives him fodder to again attack her in a primary.

Democrats also mention outgoing Sen. Mark BegichMark BegichPerez creates advisory team for DNC transition The future of the Arctic 2016’s battle for the Senate: A shifting map MORE (D-Alaska) as a candidate who could compete with her and Miller in a theoretical three-way race. She’s unlikely to lose a reelection bid — but her race is one to closely watch.