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.
And Senate Democratic Leader Harry ReidHarry ReidThe DC bubble is strangling the DNC Dems want Sessions to recuse himself from Trump-Russia probe Ryan says Trump, GOP 'in complete sync' on ObamaCare 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.
Sen. Ron JohnsonRon JohnsonOvernight Healthcare: GOP governors defend Medicaid expansion GOP senator: Let's work with Dems to 'fix' ObamaCare Right renews push for term limits as Trump takes power 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.
If Johnson isn’t the most endangered GOP Senate incumbent, it is Sen. Mark KirkMark KirkGOP senator: Don't link Planned Parenthood to ObamaCare repeal Republicans add three to Banking Committee Juan Williams: McConnell won big by blocking Obama 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.
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 GraysonAlan GraysonWhy Republicans took aim at an ethics watchdog Could bipartisanship rise with Trump government? Schumer under pressure to add Sanders to leadership team 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 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.
Sen. Rob PortmanRob PortmanGOP governors defend Medicaid expansion Senators introduce dueling miners bills Schumer puts GOP on notice over ObamaCare repeal 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.
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.
Sen. Kelly AyotteKelly AyotteTen rumored Trump Cabinet picks who didn't get a job Sasse, Perdue join Armed Services Committee Avid pilot among GOP senators joining Transportation committee 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 looks like the only other realistic pick-up opportunity for Republicans in 2016 beyond Nevada.
The GOP believes Sen. Michael BennetMichael BennetOvernight Finance: Scoop – Trump team eyes dramatic spending cuts | Treasury pick survives stormy hearing Mnuchin: Debt limit increase important, unclear on 'clean' hike Live coverage: Senators grill Trump's Treasury pick 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.
Sen. Richard BurrRichard BurrSenators introduce dueling miners bills Trump education pick to face Warren, Sanders Senate Intel panel to probe Trump team's ties to Russia MORE (R-N.C.) has yet to attract a challenger, but Democrats believe former Sen. Kay HaganKay Hagan Former Sen. Kay Hagan in ICU after being rushed to hospital GOP senator floats retiring over gridlock 2016’s battle for the Senate: A shifting map 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.
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.