This article originally appeared on The Hill.
Next year's midterm elections will provide Republicans with a major opportunity to build their majority in the Senate — if they can overcome President Trump's dismal approval ratings and internal party rancor.
The 2018 Senate map heavily favors Republicans, who will only defend eight seats, just two of which are considered vulnerable right now. By comparison, Democrats have to defend 25 seats, including 10 in states that Trump won in 2016.
Despite the rough landscape, Democrats received a handful of breaks in the early months of 2017. But with so few offensive opportunities, Democrats will have to thread the needle in deep-red states just to tread water.
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Here are the seven most vulnerable Senate seats of the 2018 midterm elections as they stand now.
1. Sen. Dean Heller (R-Nev.)
Heller is the only Republican up for reelection in 2018 who represents a state that Hillary Clinton won last November. Making matters worse for Heller, he's had a rough 2017.
His public wavering during the recent healthcare debate will hurt Heller in a state he won with only 46 percent of the vote in 2012, especially since his state accepted ObamaCare's Medicaid expansion. By the end of the GOP ObamaCare repeal push, a poll from the Democratic-leaning Public Policy Polling found him with a 22 percent approval rating.
Democrats are largely falling in line behind Rep. Jacky Rosen (D-Nev.), a strong challenger with backing from former Senate Majority Leader Harry Reid, the dean of Nevada Democrats. Rep. Dina Titus (D-Nev.) is still weighing a bid, but Democrats' near-uniform support for Rosen could squeeze Titus out of the race and give the party more time to focus on Heller.
In the meantime, Rosen and Democratic groups have hammered Heller on his healthcare position. Perennial candidate Danny Tarkanian will challenge Heller in the GOP primary from the right, a fight that could hurt Heller with the Republican base. But as an incumbent, Heller will have heavy reinforcements from Washington Republicans who are committed to helping one of their own.
2. Sen. Claire McCaskill (D-Mo.)
McCaskill is one of the few statewide Democratic officeholders left in Missouri, which Trump won by 19 points in November.
The two-term senator spent 2016 as one of Hillary Clinton's top surrogates, mounting a strong defense of the Democratic presidential nominee that McCaskill's rivals believe will become a weakness in her own campaign.
McCaskill's party also just lost Missouri's 2016 Senate race, despite a strong campaign from Democrat Jason Kander.
Missouri Attorney General Josh Hawley (R) continues to move closer to a bid, as key state Republicans push to unite around him after he won his 2016 election with more votes than Trump. And he'll likely have some serious financial backing — Politico reported last week that the Club for Growth has banked $10 million to support Hawley.
But Hawley is new to elected office, and McCaskill is a strong fundraiser. She pulled in more than any of the other vulnerable Democratic incumbents in the second quarter of 2017, leaving her with $5.1 million in the bank.
3. Sen. Joe Donnelly (D-Ind.)
Donnelly leveraged a bipartisan approach to politics and his blue-collar appeal into a Senate seat in 2012, but he'll have to fight yet another tough battle if he wants to keep his Senate seat in a state Trump won by 19 points.
Republican Reps. Todd Rokita and Luke Messer are engaged in a nasty primary. Messer is considered the more establishment Republican option, but Rokita surprised many observers with a strong second fundraising quarter where he outraised Messer by more than $400,000 and topped him in cash on hand.
The brutal primary could play to Donnelly's favor, sapping resources from the eventual Republican nominee and leaving wounds that could take some time to heal. But Republicans are seizing on a recent report that Donnelly owned stock in a family company that outsources jobs to Mexico, even as he publicly criticized other companies for sending jobs out of the country.
4. Sen. Jeff Flake (R-Ariz.)
Flake wouldn't rank this high in a normal election year, but a Republican president threatening to support a primary challenge to an incumbent GOP senator isn't normal, either.
The Arizona senator's path to reelection has grown more difficult as his feud with Trump heats up. Earlier in August, Trump called Flake "toxic" in a tweet where he applauded Kelli Ward, Flake's primary opponent. And the president's upcoming Tuesday rally in Arizona could give him a stage to take more shots at Flake or even officially endorse a primary opponent.
Complicating Flake's path is Democratic Rep. Kyrsten Sinema, who confirmed this week that she's still considering a bid.
Flake will have the backing of the National Republican Senatorial Committee and a score of outside groups that look to protect GOP incumbents, whether or not they have Trump's support. And Republicans have so far been successful at keeping a statewide Democratic push at bay. But keep an eye on how far Trump is willing to go in the primary — that could be a major factor.
5. Sen. Joe Manchin (D-W.Va.)
In 2016, West Virginia went for Trump by a larger margin than any other state — 42 points.
That resounding victory hasn't been lost on Manchin, who's made it a point to try to work with the president and Senate GOP colleagues. Another factor counting in his favor: Manchin's consistent record of winning statewide office in the West Virginia since 2001, despite the state's rightward drift.
But Trump's popularity in West Virginia has drawn interest from strong Republican candidates, and the winner will pose a serious threat to Manchin's political future. And Democrats can't be encouraged by Gov. Jim Justice's decision to ditch them and become a Republican last month, a decision he announced with Trump at his side.
Rep. Evan Jenkins (R-W.Va.) and West Virginia Attorney General Patrick Morrisey (R) are locked in another tough primary, as each tries to capture the mantle of being the "true conservative" in the race. The gloves have come off as Jenkins tries to frame Morrisey as late to the Trump bandwagon, while Morrisey focuses on Jenkins's history as a Democratic state legislator.
6. Sen. Heidi Heitkamp (D-N.D.)
Heitkamp hails from another deep-red state — North Dakota went for Trump by about 36 points.
Like Manchin, Heitkamp has walked the line between the party's opposition to Trump and his popularity among her constituents — briefly floated as a potential Agriculture Secretary, Heitkamp, like Manchin, voted for more than two-thirds of Trump's Cabinet appointments.
While Heitkamp hasn't announced whether she'll run for reelection, her steady fundraising pace keeps the door for another bid wide open. Heitkamp had $3 million in the bank as of the end of June — more than half the total she spent during her entire 2012 bid.
Heitkamp received her first challenger this week when state Sen. Tom Campbell, who is framing himself as an outsider who better exemplifies the values of voters in the conservative state, entered the race. Campbell might not have the primary field to himself, though — Rep. Kevin Cramer (R) is mulling a bid of his own.
7. Sen. Jon Tester (D-Mont.)
Republicans have a lot to like about their Montana prospects. Trump won the state by more than 20 points in November, and Republican Greg Gianforte just won the state's at-large congressional seat in a special election — even after he assaulted a reporter.
Tester has never hit 50 percent in either of his two Senate bids, winning each election with 49 percent after a libertarian candidate siphoned off part of the vote.
But Tester, a former chairman of the Democratic Senatorial Campaign Committee, is no stranger to a tough fight and has forged his own brand in the state separate from the national Democratic Party that's allowed him to outperform a typical Democratic candidate.
Republicans are going into battle without the candidates they wanted, as former Rep. Ryan Zinke (R-Mont.) is now secretary of the Interior and Attorney General Tim Fox opted to run for governor instead. State Auditor Matt Rosendale appears to be the favorite in the GOP, with businessman Troy Downing and state Sen. Albert Olszewski also in the mix.
Other races to watch
There are five other Democrats running in states Trump won — Sens. Bill Nelson (Fla.), Bob Casey Jr. (Pa.), Tammy Baldwin (Wis.), Debbie Stabenow (Mich.) and Sherrod Brown (Ohio). These five do not yet seem as vulnerable as the others, but the races could move up the rankings depending on how primaries shake out.
Florida Republicans are in a holding pattern as Gov. Rick Scott (R) weighs a bid, since his entry would immediately clear the field and give Nelson a top-tier challenger. Casey and Baldwin are waiting to see how the large GOP primary fields taking them on shake out.
Musician Kid Rock is mulling a bid against Stabenow. And Brown could face a rematch against his 2012 opponent, state treasurer Josh Mandel, but some state Republicans aren't excited about his campaign and are looking for other options.
Commentary by The Hill's Ben Kamisar and Lisa Hagen.
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