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 ReidThe Hill's Morning Report - Presented by Tax March - CDC in limbo on J&J vax verdict; Rep. Brady retiring Biden to tap Erika Moritsugu as new Asian American and Pacific Islander liaison White House races clock to beat GOP attacks 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 KirkDuckworth announces reelection bid Brave new world: Why we need a Senate Human Rights Commission  Senate majority battle snags Biden Cabinet hopefuls 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 BustosGOP campaign chief confident his party will win back House The Hill's Morning Report - Presented by the National Shooting Sports Foundation - At 50 days in charge, Democrats hail American Rescue Plan as major win House Democrats' campaign arm lifts ban on consultants who work for primary challengers MORE (Ill.) and Bill FosterGeorge (Bill) William FosterLawmakers demand justice for Adam Toledo: 'His hands were up. He was unarmed' Lawmakers say manufacturers are in better position to handle future pandemics Lawmakers grill NSA on years-old breach in the wake of massive Russian hack MORE (Ill.) as potential challengers.

 

2. Sen. Ron JohnsonRonald (Ron) Harold JohnsonPelosi: Dropping 9/11-style Jan. 6 commission an 'option' amid opposition Wisconsin state Treasurer Sarah Godlewski launches Senate bid Biden picks vocal Trump critics to lead immigration agencies 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 ManchinOn The Money: Moderates' 0B infrastructure bill is a tough sell with Democrats | Justice Dept. sues Trump ally Roger Stone for unpaid taxes Moderates' 0B infrastructure bill is a tough sell with Democrats 'Just say no' just won't work for Senate Republicans 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 AyotteOvernight Defense: NATO expanding troops in Iraq Overnight Defense: New START extended for five years | Austin orders 'stand down' to tackle extremism | Panel recommends Biden delay Afghanistan withdrawal Study group recommends Biden delay Afghanistan withdrawal 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 BurrA proposal to tackle congressional inside trading: Invest in the US Former Gov. Pat McCrory enters GOP Senate race in North Carolina Lara Trump leads GOP field in North Carolina Senate race, poll shows 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 HaganBiden's gun control push poses danger for midterms The two women who could 'cancel' Trump 10 under-the-radar races to watch in November 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 BennetDemocrats get good news from IRS Senators press for answers in Space Command move decision Biden announces first slate of diverse judicial nominees 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 UdallKennedy apologizes for calling Haaland a 'whack job' OVERNIGHT ENERGY: Haaland courts moderates during tense confirmation hearing | GOP's Westerman looks to take on Democrats on climate change | White House urges passage of House public lands package Udalls: Haaland criticism motivated 'by something other than her record' MORE (D-Colo.), defeated by Sen.-elect Cory GardnerCory GardnerBiden administration reverses Trump changes it says 'undermined' conservation program Gardner to lead new GOP super PAC ahead of midterms OVERNIGHT ENERGY: Court rules against fast-track of Trump EPA's 'secret science' rule | Bureau of Land Management exodus: Agency lost 87 percent of staff in Trump HQ relocation | GM commits to electric light duty fleet by 2035 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 RubioHillicon Valley: Biden administration sanctions Russia for SolarWinds hack, election interference Senators reintroduce bill to block NATO withdrawal New US sanctions further chill Biden-Putin relations 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 PortmanKellyanne Conway joins Ohio Senate candidate's campaign OVERNIGHT ENERGY: Senate confirms Mallory to lead White House environment council | US emissions dropped 1.7 percent in 2019 | Interior further delays Trump rule that would make drillers pay less to feds Senate confirms Biden's pick to lead White House environmental council 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 MurkowskiModerates' 0B infrastructure bill is a tough sell with Democrats The Hill's Morning Report - Presented by Tax March - CDC in limbo on J&J vax verdict; Rep. Brady retiring Trump mocks Murkowski, Cheney election chances 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 BegichAlaska Senate race sees cash surge in final stretch Alaska group backing independent candidate appears linked to Democrats Sullivan wins Alaska Senate GOP primary 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.