Math, Trump shape the developing races for U.S. Senate in 2018

The battle for the Senate in 2018 is caught between two opposing forces: math and President Donald 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 re-election 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' intraparty war can give them cover to get 10 vulnerable senators re-elected 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.

The top 10 most competitive Senate seats of the 2018 election cycle, ranked least likely to flip parties (10) to most likely (1).

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, a Republican, re-elected to a third term.

If Gov. Rick Scott, a Republican, decides to challenge Democratic Sen. Bill Nelson — as he's widely expected to — this race could be one of the most competitive (and expensive) in the nation. Scott has won re-election 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 (Democratic held)

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 have 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 endorsed by a pro-Trump Steve Bannon group. Unlike the other states on this list, Tester isn't the only statewide Democrat in office. Gov. Steve Bullock, a Democrat, won re-election last November.

7. North Dakota (Democratic held)

Sen. Heidi Heitkamp is the only Democrat to hold statewide office here. And she'll be running for re-election 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 with him 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.

Sen. Sherrod Brown's supporters say the Democrat's unique mix of anti-trade, pro-union politics fits the state's Trump-friendly 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. 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 (Democratic held)

It's hard to find a senator of any party with a stronger in-state brand than Sen. Joe Manchin, a Democrat 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 re-election 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 that he's switching parties. Like many Democrats on this list, Manchin is helped out by the fact that Republicans are fighting over who gets to challenge him.

4. Arizona (Republican held)

Republican Sen. Jeff Flake said in October that he won't seek re-election. He would have been considered vulnerable 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.

Indiana voters elected Trump by nearly 19 percentage points. That helps explain why Sen. Joe Donnelly, a Democrat, 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 that 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 a vicious primary battle.

2. Nevada (Republican held)

Sen. Dean Heller is certainly the most vulnerable Republican up for re-election next year. It's possible he's also one of the most vulnerable senators, period. He's the only Republican up for re-election in a state that 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 Affordable Care Act repeal bill. Heller's supporters say he knows the state, is known in the state and knows how to pull out a tough win.

1. Missouri (Democratic held)

Missouri has sharply turned to the right in recent years. After GOP Rep. Ann Wagner decided not to challenge Democratic Sen. Claire McCaskill, newly elected Attorney General Josh Hawley said that he will. With a strong challenger in a state that went for Trump by nearly 19 percentage points, McCaskill could have the toughest fight of any senator up for re-election.