My guess:
Rationale: Trump's approval ratings will be 45-48%. A lot of this means that Republican turnout will be relatively depressed compared to 2016. Additionally, let's go down the races. The economy will begin slowing down but I don't think it will be horribly crappy at this point.
Dem Holds
1.
Ohio: Sherrod Brown is an avowed populist who fits the state well in that regard. He'll win even though Trump won by 8 points. Any other Democrat, I'd peg this as a possible GOP pickup.
2.
Florida: Bill Nelson has always won handily in a General Election and is an inoffensive Democrat. Again, D win. Trump won FL by just 1-1.5% so it's not much of a shift to keep it Democratic.
3.
Pennsylvania. Bob Casey will prevail. Again, narrow GOP win 2016. This is a two term Democratic Senator who has experience holding down Pennsylvania and won his last term by 6%. Also, he's from a political dynasty who is legend in Pennsylvania. He's also got the advantage of being the out party. Philadelphia + the suburbs will vote Democratic while GOP turnout in the rural areas will be slightly down compared to 2016.
4.
Michigan: Debbie has been in power since 2000 and has forged a long time connection to Michigan voters. The state went Republican by just 10,000 votes. She'll be fine.
5.
Montana: Jon Tester lost his major GOP opponent in Ryan Zinke, who is now Secretary of the Interior. Tester is a decent retail politician who won re-election even as Romney won the state. Without a presidential election, Tester can probably galvanize enough of his supporters to back him for another term, especially given Montana's reservoir of Democratic votes that exist (narrow 2008 win for McCain, the state's governor is a Democrat, etc).
5.
Wisconsin. She's got a tough race, given how red Wisconsin has trended and she's a first term senator unlike Debbie Stabenow. She's also a gay woman and while that wasn't a huge issue in 2012, this could be a problem in 2018. The GOP has kept a presence in Wisconsin and Walker will be running for a third term. This is kind of a 52-47% Democratic win, I think. But I could see WI going GOP. We'll see what the national moods are.
GOP Holds
1.
Arizona: Jeff Flake has staked himself out as a critic of Donald Trump and yet also has staked himself out as a libertarian conservative. I don't think Arizona's ready to make the big jump to electing a Democratic Senator - yet. Flake holds on by a similar 50-45% to his 2012 win.
2.
Nevada: Heller has a unparalleled ability to survive in rough terrain. He beat the Reid machine in 2012 and even as Obama won Nevada by six points, Heller has survived. Nevertheless, this is one of the races I'm least sure about. But I think Heller will be able to squeak it out.
3.
Texas. Not ready to be Atlas Red, yet. Cruz will win renomination and the General.
GOP pickups
1.
West Virginia: The state's been trending hard right and while I expect it to backtrack to the left in the 2020s as the Bernie Sanders Democrats come to power, I think that the current trends suggest that West Virginians will junk Manchin. Nevertheless, could see Wisconsin sending Baldwin home and keeping Manchin. Manchin is the most skilled Democratic pol in a red state.
2.
Missouri. Claire McCaskill has a major target on her back. She was due to lose in 2012 but eked out a win because of Akin. I don't think that Missouri will be that forgiving this time around.
3.
Indiana: Same deal as MO. The GOP will be more careful to not nominate a Murdoch. Trump also won the state by 20 points.
4.
North Dakota. See Indiana and Missouri and Wisconsin. Heitkamp is a first term senator who won by 1 point in a red state and expect the state to boot her. She's also lost a race here in North Dakota, suggesting vulnerability.
so, buckets - the deep Republican states (Atlas blue) will elect GOP senators, swing and lean Dem states will elect Dem Senators.
GOP gains +4 even as they lose 4-10 seats in the House. The major lesson is that the polarization continues apace as GOP states start moving more in line and Democratic states remain committed to the Dem Party and the swing states go to the party out of power. Don't think there will be a scandal big enough for Trump voters to stay home in 2018 (think that's more after 2018).
