The 10 Senate seats most likely to flip in 2018: February edition

Key races to watch in the Senate in 2018
Key races to watch in the Senate in 2018

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    Key races to watch in the Senate in 2018

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Key races to watch in the Senate in 2018 02:16

Washington (CNN)Democratic incumbents, challengers and open-seat contenders outraised Republicans across the Senate map in 2017, a new round of campaign finance reports show.

The fundraising gap is yet another sign that the GOP is struggling to take advantage of a map ripe with Democratic incumbents in red states.
The good news for Republicans: Their tax bill is more popular than it was weeks ago, polls show. And Vice President Mike Pence showed some teeth on a trip to West Virginia, previewing a more aggressive line of attack from President Donald Trump's administration against Democratic incumbents.
In this year's midterm elections, Democrats -- or independents who caucus with them -- are defending 26 seats, while Republicans are defending just eight. The GOP has a 51-49 advantage in the Senate at the moment.
    Here are 10 Senate seats most likely to switch party hands in this year's midterms, as of February 2018:

    1. Nevada

    Incumbent: Republican Sen. Dean Heller
    Heller raised just $820,000 in 2017's fourth quarter. That's bad -- particularly for an incumbent.
    The Democratic candidate, Rep. Jacky Rosen, hauled in more than $1.5 million, helping her close Heller's $4.2 million to $1.8 million cash on hand advantage. And Heller still has to contend with Republican primary challenger Danny Tarkanian.
    This is the only state on this list that Hillary Clinton won in 2016. It's diverse, Democrats there are organized and Heller's gymnastics on health care are sure to provide fodder for attack ads targeting him.

    2. Arizona

    Open seat (Republican Sen. Jeff Flake is retiring)
    GOP Rep. Martha McSally is in a tough spot when it comes to Trump.
    She told the Los Angeles Times it's "not your business" whether she voted for him in 2016, and she recently attended a fundraiser hosted by Randy Kendrick, who funded a "Never Trump" super PAC. But she's also backing a hard-line GOP approach to immigration.
    McSally is trying to navigate through a primary against state Sen. Kelli Ward and former Maricopa County sheriff Joe Arpaio while preserving some room to pivot for the general election.
    The biggest problem for Republicans here is that the primary is August 28 -- which means the winner won't have much time to turn to Democratic Rep. Kyrsten Sinema (who now has $5.1 million in the bank) before November.

    3. Missouri

    Incumbent: Democratic Sen. Claire McCaskill
    Is Josh Hawley ready for prime time?
    The GOP's prized recruit of the 2018 cycle raised just shy of $1 million in 2017's fourth quarter and has $1.2 million on hand -- far short of McCaskill's $2.9 million quarter and $9 million war chest.
    The 38-year-old Missouri attorney general was also taped blaming the cultural revolution of the 1960s for sex trafficking -- an odd linkage that's shaken up the race in recent days.
    It led Democrats to chirp that Hawley demonstrated why he's a less frequent presence on the campaign trail than McCaskill, who held 51 town hall events last year.
    Still, Trump won Missouri by 19 percentage points. That's a big hole for McCaskill to climb out of.

    4. Indiana

    Incumbent: Democratic Sen. Joe Donnelly
    Republicans are split over whether they really want to brand Donnelly as "Mexico Joe."
    The national GOP loves blasting Donnelly over reports that a family business outsourced jobs to Mexico so much that the National Republican Senatorial Committee sent a mariachi band to troll him at a campaign stop in August.
    But Gov. Eric Holcomb and the Indiana Republican Party saw the stunt as offensive and aren't taking part in the line of attack.
    Republicans face a bitter, competitive primary between Reps. Todd Rokita and Luke Messer and former state Rep. Mike Braun that won't be settled until May.
    In the meantime, it's worth paying close attention to whether poll numbers for the GOP's tax bill continue to rise. At the Republican National Committee's winter meeting in Washington, one Hoosier said Donnelly could have iced this race by voting for the tax bill, and speculated that he'll ultimately regret his vote against it.

    5. West Virginia

    Incumbent: Democratic Sen. Joe Manchin
    Manchin might have come out on top of his recent rumble with Vice President Mike Pence when he labeled Pence's attacks on his vote against the tax bill "why Washington Sucks." (Yes, he capitalized the S.)
    But Pence sent an important signal by hitting Manchin hard in his home state: The Trump administration is ready and willing to burn its bridges with even the friendliest Democratic senators in order to take their seats this fall.
    Manchin, after all, stood and applauded during Trump's State of the Union when other Democratic senators sat on their hands.
    This is another race where a crowded, nasty, personal GOP primary will sort itself out in May. Rep. Evan Jenkins, Attorney General Patrick Morrisey and former coal baron Don Blankenship -- who was in prison just last year for conspiracy to commit mine safety violations -- are all running.

    6. North Dakota

    Incumbent: Democratic Sen. Heidi Heitkamp
    The fourth in our run of five consecutive Democratic-held seat where Trump won by double digits in 2016, this race just got a little more crowded with former state GOP chair Gary Emineth entering the race. He joins state Sen. Tom Campbell in a Republican contest that the national GOP is disappointed Rep. Kevin Cramer decided not to enter.
    Remember Trump calling Heitkamp a "good woman" last year, on stage with the television cameras rolling? It's another reason why it was so important for Pence to show he's willing to give Republicans much tougher tape to use against Democratic incumbents.

    7. Montana

    Incumbent: Democratic Sen. Jon Tester
    Tester was the only one of the five Democratic incumbents in states Trump won by double digits to vote against the continuing resolution that ended the government shutdown.
    It was a risky vote, sure -- but also a reflection of the confidence Democrats have that he can hold this seat against the likes of state auditor Matt Rosendale or businessman Troy Downing.

    8. Wisconsin

    Incumbent: Democratic Sen. Tammy Baldwin
    Baldwin is a top target of the Koch brothers, who plan to spend $400 million in this year's midterms. They'll try to use her vote against the tax bill to portray Baldwin as too liberal. And with Gov. Scott Walker up for re-election, his effective state party machinery will be running at full steam, benefiting Republicans up and down the ballot.
    State Sen. Leah Vukmir and businessman Kevin Nicholson are in a competitive (and likely expensive) GOP primary.

    9. Florida

    Incumbent: Democratic Sen. Bill Nelson
    Wake us up when Rick Scott makes up his mind.
    The term-limited governor currently in his last year could announce whether he'll run for the Senate in March, after Florida's state legislative session wraps up and just before the filing deadline.
    If he's in, this is a marquee race and Democrats might have to spend heavily to defend the seat. If he's out, that money could be scattered to the states that are higher on the list -- where the media markets are smaller and the advertising dollar goes much farther.

    10. Ohio

    Incumbent: Democratic Sen. Sherrod Brown
    State treasurer Josh Mandel's surprise withdrawal left Republicans with Rep. Jim Renacci and investment banker and major donor Mike Gibbons to take on the populist progressive Brown, who has $10 million in the bank and a little 2020 presidential buzz following him.
    This is a state Trump won by 9 percentage points in 2016 -- so a rise in Trump's approval rating would make Brown uncomfortable.

    Just missed

    Increasingly, it looks like Texas -- not Tennessee -- represents Democrats' best chance at putting a third GOP-held seat in play.
    Democratic Rep. Beto O'Rourke actually outraised Texas Sen. Ted Cruz in 2017's fourth quarter, hauling in $2.4 million to Cruz's $1.9 million. That means Cruz's cash on hand advantage is down to $7.3 million to O'Rourke's $4.6 million -- nowhere near the insurmountable gap many expected.
    Hotline's Josh Kraushaar convincingly made the case for a close Texas race in part by pointing to three competitive House contests that will engage voters in the Dallas, Houston and San Antonio media markets.
    Yes, there's a Lucy-with-the-football element at play for Democrats in Texas. For years, they've thought they could win there -- and for years, their best candidates ultimately haven't even been able to make Republicans sweat. But elections in recent months have shown that diverse states with expansive suburbs are rapidly moving leftward. It makes Texas one to watch closely.
    In Tennessee, we're still waiting to see whether former Gov. Phil Bredesen has shaken off the dust and is headed into an open-seat contest where he'll likely face Rep. Marsha Blackburn.
    In Pennsylvania, Republicans are concerned about the campaign of Rep. Lou Barletta -- a Trump ally in a year when that's a politically problematic label. And in Minnesota, newly appointed Sen. Tina Smith has not yet drawn a high-profile Republican opponent. State Sen. Karin Housley looks like the leading GOP contender.