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 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 (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 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 (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 Bustos Cheryl (Cheri) Lea BustosThe Hill's Campaign Report: Biden asks if public can trust vaccine from Trump ahead of Election Day | Oklahoma health officials raised red flags before Trump rally DCCC dropping million on voter education program Clark rolls out endorsements in assistant Speaker race MORE (Ill.) and Bill Foster George (Bill) William Foster81 Nobel laureates endorse Biden's White House bid Trump payroll tax deferral finds few takers among businesses Former Obama Ebola czar Ron Klain says White House's bad decisions have put US behind many other nations on COVID-19; Fears of virus reemergence intensify MORE (Ill.) as potential challengers.

2. Sen. Ron Johnson Ronald (Ron) Harold JohnsonThe Hill's 12:30 Report: Ginsburg lies in repose CHC leaders urge Senate to oppose Chad Wolf nomination Top GOP senators say Hunter Biden's work 'cast a shadow' over Obama Ukraine policy 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 Manchin Joseph (Joe) ManchinManchin defends Supreme Court candidate Barrett: 'It's awful to bring in religion' The Hill's Morning Report - Sponsored by Facebook - Trump, GOP allies prepare for SCOTUS nomination this week Trump meets with potential Supreme Court pick Amy Coney Barrett at White House 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 Ayotte Kelly Ann AyotteBottom line Bottom line Bottom Line 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 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 (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 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 (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 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 (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 Udall Mark Emery UdallThe 10 Senate seats most likely to flip Democratic presidential race comes into sharp focus Democrats will win back the Senate majority in 2020, all thanks to President Trump MORE (D-Colo.), defeated by Sen.-elect Cory Gardner Cory Scott GardnerCook Political Report shifts Colorado Senate race toward Democrat Overnight 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 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 Rubio Marco Antonio RubioOvernight Defense: Pentagon redirects pandemic funding to defense contractors | US planning for full Afghanistan withdrawal by May | Anti-Trump GOP group puts ads in military papers Democrats step up hardball tactics as Supreme Court fight heats up Press: Notorious RBG vs Notorious GOP 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 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)

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 Murkowski Lisa Ann MurkowskiClub for Growth to spend million in ads for Trump Supreme Court nominee Pebble Mine CEO resigns over secretly recorded comments about government officials Maryland's GOP governor says Republicans shouldn't rush SCOTUS vote before election 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 Begich Mark Peter BegichAlaska group backing independent candidate appears linked to Democrats Sullivan wins Alaska Senate GOP primary Alaska political mess has legislators divided over meeting place 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.