The battle for the Senate in 2018 is caught between two opposing forces: math and President Trump.

The math looks terrible for Democrats. Republicans are defending nine seats next November compared to 25 for Democrats. Ten Senate Democrats are attempting to win reelection in states Trump won. Five of those states he won by double digits.

On the Trump side, Democrats are hoping the president's unpopularity coupled with Republicans' intra-party war can give them cover to get 10 vulnerable senators reelected and maybe knock out a couple of Republicans. Unless something really major happens, control of the Senate isn't in play (Republicans control it by a slim margin of 52-48), but both sides have a chance to pick up seats that have eluded them for years.

Anyway. Here is The Fix's first ranking of the the top 10 most competitive Senate seats of the 2018 election cycle. We've ranked them least likely to flip parties (10) to most likely (1).  We'll update the rankings semi-regularly on Fridays all the way to Election Day.

10. Wisconsin (Democratic held): Sen. Tammy Baldwin is trying to stay the only statewide Democrat in Wisconsin. She's a formidable enough candidate, able to speak to Republican-leaning voters. But she'll also be running against one of the best-mobilized state parties in all of politics, which will be trying to get Gov. Scott Walker (R) reelected to a third term.


Sen. Tammy Baldwin (D-Wis.) EPA/MICHAEL REYNOLDS

9. Florida (D): If Gov. Rick Scott (R) decides to challenge Sen. Bill Nelson (D) — as he's widely expected to — this race could be one of the most competitive (and expensive) in the nation. Scott has won reelection twice and has millions of his own money he can put into the race. Democrats see vulnerability following Hurricane Irma, especially after 12 patients at a South Florida nursing home died when the power went out; Republicans disagree that Scott will be blamed for that.

8.Montana (D): Montana is the first of several states on this list that Trump won by at least 20 percentage points. And yet, Sen. Jon Tester is the first of several Democrats on this list who has successfully adapted to fit the state's unique politics. He's voted for more than half of Trump's nominees, and on the weekends, he farms on land his grandparents homesteaded a century ago. Republicans are tangled in a four-way primary on who gets to challenge him. Auditor Matt Rosendale was just endorsed by a pro-Trump Stephen K. Bannon group. Unlike the other states on this list, Tester isn't the only statewide Democrat in office. Gov. Steve Bullock (D) won reelection in November by 4 points.


With the help of his wife, Sharla Tester, Sen. Jon Tester repairs his cultivator on his 1,800 acre farm in Big Sandy, Mont. in April. (Melina Mara/The Washington Post)

7. North Dakota (D): Sen. Heidi Heitkamp is the only Democrat to hold statewide office here. And she'll be running for reelection in a state that Trump won by 35 points. Trump himself gift-wrapped some help for Heitkamp when he visited North Dakota in September, had her on stage and called her a "good woman." Republicans dominate the state, but their primary field is still developing. Wealthy GOP state Sen. Tom Campbell is the only declared GOP candidate, but U.S. Rep. Kevin Cramer could get in, too, dividing the party.

6. Ohio (D): Sen. Sherrod Brown's (D) supporters say his unique mix of anti-trade, pro-union politics fits the state's Trump-friendly profile profile. But Ohio just may be turning too red. Some Democrats say the state's pro-Trump Appalachia region will be a force for Brown to reckon with. Trump won Ohio by eight points. And Brown is the only Ohio Democrat to win statewide in a decade. He will likely have a rematch against State Treasurer Josh Mandel, whom he beat by six points in 2012.

5. West Virginia (D): It's hard to find a senator of any party with a stronger in-state brand than Sen. Joe Manchin (D), who has been leading the state one way or another for more than a decade. And yet it's also hard to find a Democratic senator up for reelection in a more pro-Trump state. Trump won West Virginia by more than 40 percentage points. The state's newly elected Democratic governor recently announced at a Trump rally he's switching parties. Like many Democrats on this list, Manchin is helped out by the fact Republicans are fighting over who gets to challenge him.

4. Arizona (Republican held): Sen. Jeff Flake is one of two Republicans on this list. And he's near the top of it for a few unique reasons: Arizona is shaping up to be ground zero for Republicans' war with itself, a war Flake arguably brought on himself when he wrote a book criticizing Trump. Trump supporters are backing Flake primary challenger Kelli Ward, and Trump has said he's happy she's in the race. Meanwhile, Democrats recruited Rep. Kyrsten Sinema. They say her moderate, even GOP-friendly politics match the state's.

3. Indiana (D): Indiana voters elected Trump by nearly 19 points. That helps explain why Sen. Joe Donnelly (D) was eager to appear on stage with Trump when he visited in September and share with voters how many times he's been to the White House to talk jobs with the president. As Donnelly rails against outsourcing, Republicans plan to hit him hard with the fact his family's arts and crafts company has a plant in Mexico. Donnelly gets some breathing room while two House Republicans, Luke Messer and Todd Rokita, are locked in one of the most vicious primary battles of any Senate race this election cycle.

2. Nevada (R): Dean Heller is certainly the most vulnerable Republican up for reelection next year. It's possible he's also one of the most vulnerable senators, period. He's the only Republican up for reelection in a state Hillary Clinton won. Like Flake, he's forced to fight off both sides, a primary challenger in Danny Tarkanian who says he's not pro-Trump enough, and a Democratic challenger in first-term Rep. Jacky Rosen, who says he's too pro-Trump. Democrats think Heller has upset his base too much by voting against an Obamacare repeal bill. Heller's supporters say he knows the state, is known in the state and knows how to pull out a tough win.


President Trump speaks with Sen. Dean Heller in July. (Jabin Botsford/The Washington Post)

1. Missouri (D): Missouri has sharply turned to the right in recent years. To win reelection in 2012, Sen. Claire McCaskill (D) tinkered with Republicans' primary to get a challenger who used the term "legitimate rape." After GOP Rep. Ann Wagner decided not to challenge her, newly elected Attorney General Josh Hawley announced this week he will. Republicans say Hawley is the real deal — a young, telegenic, smart campaigner who can unite Trump Republicans and establishment Republicans. Democrats say Hawley's bridge-building pitch can be sullied quickly as a ladder-climber, and they're skeptical such an untested candidate with establishment backing can even win the primary.

But with a potentially stronger challenger in a state that went for Trump by nearly 19 percentage points, McCaskill could have the toughest fight of her career — and of any senator up for reelection in November.