Democrats have a great chance to win back the Senate in 2016 after losing their majority last fall.

Republicans must defend 24 seats, compared with only 10 for Democrats, and they must do so in a presidential election year when turnout could favor their opponents.

Retirements have made the GOP’s path even tougher.

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Sen. Marco Rubio Marco Antonio RubioSunday shows preview: Justice Ginsburg dies, sparking partisan battle over vacancy before election Florida senators pushing to keep Daylight Savings Time during pandemic Hillicon Valley: DOJ indicts Chinese, Malaysian hackers accused of targeting over 100 organizations | GOP senators raise concerns over Oracle-TikTok deal | QAnon awareness jumps in new poll MORE (R-Fla.) would have been the favorite in Florida but has decided to run for president rather than seek reelection. In Indiana, Republicans still have a great chance to hang on to Sen. Dan Coats Daniel (Dan) Ray CoatsFBI chief says Russia is trying to interfere in election to undermine Biden The Hill's Morning Report - Sponsored by The Air Line Pilots Association - Trump, Biden renew push for Latino support Former Intel chief had 'deep suspicions' that Putin 'had something on Trump': book MORE’s seat, but his retirement will make the task more difficult.

And Senate Democratic Leader Harry Reid Harry Mason ReidGraham signals support for confirming a Supreme Court nominee this year Trump signals he will move to replace Ginsburg 'without delay' Senate Republicans signal openness to working with Biden MORE’s (Nev.) decision to not seek reelection might benefit Democrats, who will now seek to retain an open seat.

Here are The Hill’s rankings of the most competitive Senate races of the cycle based on interviews with political insiders from both parties.

Overall, 8 of the 10 most hotly contested seats are held by Republicans, many of whom represent states won by President Obama in the 2008 and 2012 elections.

WISCONSIN

Sen. Ron Johnson Ronald (Ron) Harold JohnsonThe Hill's Morning Report - Sponsored by The Air Line Pilots Association - White House moves closer to Pelosi on virus relief bill Second GOP senator to quarantine after exposure to coronavirus GOP-led panel to hear from former official who said Burisma was not a factor in US policy MORE (R-Wis.) faces the most uphill climb of any GOP Senate incumbent seeking reelection, at least for the time being.

A survey by the Democratic firm Public Policy Polling in early March showed Johnson winning 41 percent of the vote, to 50 percent for former Sen. Russ Feingold (D-Wis.).

Democrats are pressuring Feingold to seek a rematch with Johnson, who scored a 5-percentage-point victory in 2010 — a great year for Republicans nationally.

If Feingold enters the race, he’ll be an immediate favorite over Johnson, who poured millions of dollars of his own money into the 2010 race but says he won’t self-fund this time around.

Presidential coattails could be a problem for Johnson. Wisconsin hasn’t gone for a Republican presidential candidate since backing President Reagan for reelection in 1984. But if Wisconsin Gov. Scott Walker is the Republican candidate, it could help Johnson.

ILLINOIS

If Johnson isn’t the most endangered GOP Senate incumbent, it is 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 (Ill.).

Like Johnson, Kirk represents a solidly blue state and won election in a midterm year. He’ll now have to win reelection in a presidential year, where turnout could be less favorable for him.

Kirk, who suffered a stroke in 2012, is expected to face Rep. Tammy Duckworth (D-Ill.), who lost both her legs in combat in Iraq.

He’s shown some resilience, as polls show a dead-even race.

And Duckworth could face a primary challenge. Allies of President Obama are reportedly urging Urban League CEO Andrea Zopp to enter the race against Duckworth, and Rep. Robin Kelly (D-Ill.) is also still considering a bid.

FLORIDA

The battle in the nation’s largest swing-state instantly became a toss-up when Republicans lost Rubio to the presidential race.

The Democratic Senatorial Campaign Committee has already thrown its weight behind Rep. Patrick Murphy (D-Fla.), a centrist and former Republican. That’s likely to infuriate liberal firebrand Rep. 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 (D-Fla.), who is considering a bid, as well as progressive groups in the state.

Rep. Ron DeSantis (Fla.), a conservative lawmaker with Tea Party credibility, is so far the only declared candidate on the Republican side. He could still face a challenge from another member of the Florida delegation in the House, or Lt. Gov. Carlos López-Cantera (R-Fla.).

NEVADA

Nevada is the GOP’s best shot at picking up a seat currently held by Democrats.

Still, Republicans might have had a better chance if Senate Minority Leader Harry Reid (D-Nev.) hadn’t decided to retire.

Reid’s reelection bid would have created a huge target for the GOP, pulling millions into the state.

Now, Republicans will battle for an open seat.

Reid’s handpicked successor, former Nevada Attorney General Catherine Cortez Masto (D), is so far running unopposed for the Democratic nomination.

Republican Gov. Brian Sandoval (Nev.) is wildly popular and would be the favorite to win the Senate race, but he appears likely to remain in his current job.

About a half-dozen other potential GOP candidates in the state are waiting for Sandoval to officially bow out before deciding for sure whether to enter the race. Rep. Joe Heck (R-Nev.), an Iraq War veteran and proven fundraiser from a swing-district, is considered by some observers to be the strongest option.

President Obama carried the Silver State in both of his White House runs, but Nevada voters backed President George W. Bush in the two elections before that.

OHIO

Sen. Rob Portman Robert (Rob) Jones PortmanRomney undecided on authorizing subpoenas for GOP Obama-era probes Congress needs to prioritize government digital service delivery House passes B bill to boost Postal Service MORE’s (R-Ohio) campaign launch has been among the most impressive of the nascent cycle.

Portman banked nearly $3 million in the first quarter of the year, bringing his total cash on hand to $8 million. The Portman campaign has also rolled out hundreds of endorsements from Republican officials in the state, from Gov. John Kasich (R-Ohio) on down.

Still, Portman, a former Bush administration budget director and trade representative, could face a difficult reelection bid.

He’s likely to face former Gov. Ted Strickland (D-Ohio) in the general election, and the Democrat is a prodigious fundraiser with solid name recognition across the state.

A Quinnipiac University poll released in April showed Strickland with a 9-percentage-point lead over Portman, highlighting the strong headwinds Portman faces.

PENNSYLVANIA

By all accounts, Sen. Pat Toomey (R-Pa.) should be in trouble.

Toomey, a former president of the conservative Club for Growth, won his seat in the GOP landslide election of 2010. It was very favorable turf.

Presidential years in Pennsylvania have different patterns — and usually different electorates.

The state hasn’t gone for a GOP presidential nominee since 1988, and Democratic presidential candidates have consistently won the state by comfortable margins.

The problem for Democrats is that their only candidate challenging Toomey so far is former Rep. Joe Sestak (D-Pa.), the retired Navy admiral bested by the Republican in 2010.

A Quinnipiac University survey released last month showed Toomey with a 13-percentage-point lead over Sestak. Toomey also ended 2014 with $5.8 million in cash on hand, compared with $1.5 million for Sestak.

Still, Toomey only edged Sestak by 2 percentage points in Pennsylvania in 2010. That suggests Sestak could defeat Toomey if the Democratic presidential ticket has some coattails.

NEW HAMPSHIRE

Sen. Kelly Ayotte Kelly Ann AyotteBottom line Bottom line Bottom Line MORE (R-N.H.) is in a similar situation to Toomey and Portman.

New Hampshire has gone for the Democrat in the last three presidential races, and in five of the last six overall.

If a Democratic presidential candidate wins the state in 2016, Ayotte will be in for the race of her life.

Polling suggests she would be in much bigger trouble if the state’s popular Democratic Gov. Maggie Hassan enters the race.

A Dartmouth University poll released last week showed Ayotte with an early 5-percentage-point lead over Hassan, although previous polls have shown Hassan with a small advantage.

If Hassan doesn’t run, the Dartmouth poll showed Ayotte with a 20-percentage-point lead over her next closest challenger, former Rep. Carol Shea-Porter (D-N.H.).

COLORADO

Colorado looks like the only other realistic pick-up opportunity for Republicans in 2016 beyond Nevada.

The GOP believes 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.) is vulnerable, but unseating him will be a heavy lift in a more favorable year for Democrats.

The former head of the Democratic Senatorial Campaign Committee raised more than $2 million in the first quarter, making him one of only four Senate candidates to do so.

No Republicans have announced for the race yet. Rep. Mike Coffman (R-Colo.) is expected to make his intentions known soon. Still, it’s unclear whether he intends to run, and the GOP bench falls off markedly behind him.

NORTH CAROLINA

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.) has yet to attract a challenger, but Democrats believe 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 (D-N.C.) would give Burr all he could handle if she could be persuaded to run.

Hagan lost to now-Sen. Thom Tillis (R-N.C.) by less than 2 percentage points in 2014, a wave election year for the GOP.

President Obama carried North Carolina in 2008 and narrowly lost it in 2012.

INDIANA

The Hoosier State became a more attractive target for Democrats when Sen. Dan Coats (R-Ind.) announced last month he wouldn’t seek reelection.

Former Indiana Republican Party Chairman Eric Holcomb is running for the GOP nomination, as is Rep. Marlin Stutzman (R-Ind.), a third-term lawmaker aligned with the Tea Party.

The race would instantly become a toss-up if former Sen. Evan Bayh (D-Ind.) got in.

Bayh has nearly $10 million to campaign with. It seems unlikely he will run, though, considering he decided not to seek a third term in 2010 due to his frustration with politics on Capitol Hill.

President Obama carried Indiana in 2008, but it’s generally a reliably red state in presidential years.