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 Mason ReidSenate heads home to campaign after deal on Trump nominees GOP has always been aggressive in trying to weaponize the system of judicial nominations Republicans come full circle with Supreme Court battle to the end 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 Steven KirkThis week: Trump heads to Capitol Hill Trump attending Senate GOP lunch Tuesday High stakes as Trump heads to Hill 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 BustosCheryl (Cheri) Lea BustosDemocrats are offering real change for the people Overnight Energy — Presented by Southern Utah Wilderness Alliance — Judge upholds Obama's marine monument | GOP lawmakers worried states using water rule to block fossil fuels | Lawmakers press Trump ahead of ethanol decision Hopes for infrastructure deal with Trump rise if Dems win House MORE (Ill.) and Bill FosterGeorge (Bill) William FosterCongress and Trump are out of step on intellectual property Overnight Defense: House passes 5B defense spending bill | Pentagon moving forward on Trump military parade | Mattis vows 'ironclad' support for South Korea's defense House passes 5B Pentagon spending bill MORE (Ill.) as potential challengers.

 

2. Sen. Ron JohnsonRonald (Ron) Harold JohnsonGOP senator seeking information on FBI dealings with Bruce Ohr, former DOJ lawyer Election Countdown: O'Rourke brings in massive M haul | Deal on judges lets senators return to the trail | Hurricane puts Florida candidates in the spotlight | Adelson spending big to save GOP in midterms Senate Homeland chair vents Mueller probe is preventing panel from receiving oversight answers 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 ManchinJoseph (Joe) ManchinGillibrand backs Manchin, Bredesen despite their support of Kavanaugh Senate heads home to campaign after deal on Trump nominees Overnight Energy: Climate skeptic confirmed as DOJ environmental lawyer | EPA to phase out air pollution panel | Ad campaign targets mercury rule proposal 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 Ann AyotteElection Countdown: O'Rourke brings in massive M haul | Deal on judges lets senators return to the trail | Hurricane puts Florida candidates in the spotlight | Adelson spending big to save GOP in midterms GOP mulls having outside counsel question Kavanaugh, Ford Pallbearers, speakers announced for McCain's DC memorial service and Capitol ceremony 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 Mauze BurrCollusion judgment looms for key Senate panel The National Trails System is celebrating 50 years today — but what about the next 50 years? Key conservation fund for parks set to expire 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 Ruthven Hagan2020 Dems compete for top campaign operatives Senate GOP rejects Trump’s call to go big on gun legislation Politics is purple in North Carolina 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 Farrand BennetEagles player sits out national anthem Trump administration denied it has ‘secret’ committee seeking negative information on marijuana: report Overnight Health Care: Senators target surprise medical bills | Group looks to allow Medicaid funds for substance abuse programs | FDA launches anti-vaping campaign for teens 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 Emery UdallTrump calls Kavanaugh accusations ‘totally political’ Record number of LGBT candidates running for governor Senate GOP rejects Trump’s call to go big on gun legislation MORE (D-Colo.), defeated by Sen.-elect Cory GardnerCory Scott GardnerDemocrats must end mob rule GOP senators praise Haley as 'powerful' and 'unafraid' Democrats won’t let Kavanaugh debate die 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 Antonio RubioSenators pledge action on Saudi journalist’s disappearance Senators concerned as Trump official disputes UN climate change warning Rubio: Response to death of Saudi journalist 'can't be symbolic' 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 PortmanRobert (Rob) Jones PortmanElection Countdown: O'Rourke brings in massive M haul | Deal on judges lets senators return to the trail | Hurricane puts Florida candidates in the spotlight | Adelson spending big to save GOP in midterms How Kavanaugh got the votes  Collins to support Kavanaugh, securing enough votes for confirmation 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 Ann MurkowskiMurkowski not worried about a Palin challenge Flake on Kavanaugh confirmation: To see GOP 'spiking the ball in the end zone' doesn't seem right GOP fractured over filling Supreme Court vacancies in 2020 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 Peter BegichRepublicans see silver linings in deep-blue states Election Countdown: Trump plans ambitious travel schedule for midterms | Republicans blast strategy for keeping House | Poll shows Menendez race tightening | Cook Report shifts Duncan Hunter's seat after indictment The Hill's Morning Report — Trump’s legal jeopardy mounts after Manafort, Cohen felony counts 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.