Charlie Cook writes in The National Journal, via The Cook Political Report, “while the odds are better that Republicans hold onto, rather than lose, the Senate, there is at least a 30 percent chance the Senate flips.” Noting that “Democrats need a three-seat gain to win a Senate majority if a Democrat wins the White House—four seats if they don’t,” Cook explains “if Trump loses, the GOP chances of retaining control drops to just 55 or 60 percent, or maybe even less.” Further,

…In this new hyper-partisan political climate, with very little ticket-splitting taking place, more people than ever before are voting straight-line Republican or Democrat. The 2016 election was the first in American history in which every single Senate race was won by the same party as that state voted for President. In fact, 88 out of 100 Senators are now from the same party as their state’s most recent presidential victor.

However, Cook asks, “do Democrats really need only three or four seats based on the presidential outcome, or do they need to gross four or five seats in order to net three or four?” The latter scenario makes sense, because:

It’s hard to see how Democratic Sen. Doug Jones wins reelection in a presidential year with presidential-level turnout, even if Republicans nominate their worst possible candidate, former judge Roy Moore. The accusations about Moore and young women were fresh at the time of the December 2017 special election, but it’s old news now and likely to have less saliency.

If Democrats need to win at least four seats, where do they get them? Most would put GOP incumbents Martha McSally in Arizona and Cory Gardner in Colorado at the top of the Democrats’ target. My guess is that both have about a 50-50 chance, at best, particularly if a Democrat is prevailing at the top of the ticket.

My National Journal colleagues Drew Gerber and Kyle Trygstad presented their latest Hotline’s Senate Power Rankings, sequencing the top 10 seats in order of vulnerability. More or less, I agree with their rankings and analysis, but where I most disagree is Maine, where Susan Collins is seeking reelection. Drew and Kyle put Maine behind North Carolina; I would put it ahead in vulnerability.

My view is that Collins’s chances put her just barely behind McSally and Gardner. Yes, Collins was last reelected with a very impressive 67 percent of the vote, normally a sign of great strength even six years later. But, consider first that the 67 percent was in 2014, a fabulous year for Republicans up and down the ballot. Second, Collins did extremely well among groups with whom she is unlikely to do even remotely as well this time, particularly given her support of Brett Kavanaugh’s Supreme Court nomination and vote for President Trump’s tax cuts.

The 2014 exit polls showed that Collins carried 37 percent of the vote among self-described liberals and 39 percent from Democrats. Anybody think she will remotely do that again? What about winning 69 percent of independents and 72 percent of moderates. This is not to predict that she will lose, just that this is likely to be an extremely difficult race and that it has a higher chance of going Democratic than several others.

Cook argues further,

After Arizona, Colorado, and Maine, Democrats are likely to need at least one more, and that would require a fairly substantial wave. Democrats need the suburbs to move in their direction, particularly among college-educated women, as strongly next November as last November. They need to pick up one or both of the Georgia seats—incumbent David Perdue and a seat expected to be vacated by Johnny Isakson, who is stepping down for health reasons—and/or beating Thom Tillis of North Carolina. That means suburban voters outside of Atlanta, Charlotte and the Research Triangle being as angry at Republicans as we saw in so many Southern Congressional races last year.

Or, they could pick up Iowa or an open seat in Kansas, but the latter is likely possible only if controversial former Secretary of State Kris Kobach wins the GOP nomination. Beyond that, beating John Cornyn in Texas and Senate Majority Leader Mitch McConnell in Kentucky seem a bit too far for Democrats to win this time.

Here’s a 2020 Senate Race Ratings map from Sabato’s Crystal Ball, updated August 28th.