Suppose Joe Biden is elected president in November, but the Senate stays Republican. Or, suppose he loses, but the Democrats manage to take the Senate anyway.

If Americans vote the way they have in recent elections, both those eventualities are unlikely.

As the rift between the two parties, and that between their supporters, has grown to Grand Canyon dimensions, Americans have largely abandoned ticket-splitting.

In the 2016 presidential election, every state that elected a Republican senator voted for Donald Trump for president, while every state that elected a Democratic senator voted for Hillary Clinton. Since the nation inaugurated the popular election of senators in 1913, that had never happened.

Even in midterm elections, ticket-splitting — a pervasive feature of American voting throughout the 20th century — has gone into steep decline. Of the 21 states that elected both governors and senators in 2018, only five elected one from each party. In the three states that elected Democratic senators and Republican governors, those governors — Massachusetts’ Charlie Baker, Maryland’s Larry Hogan, Vermont’s Phil Scott — had each largely repudiated President Trump and ran (and in Baker’s and Hogan’s case, already governed) as centrists.

What does this portend when considering whether Democrats have any chance of flipping the Senate in 2020?

To oust Mitch McConnell from the post of majority leader, the Dems need to concentrate on states where Republican Senate seats are up for a vote and where Biden (or whoever becomes the Democratic nominee) has a good chance of winning.

That list certainly includes Colorado, where Republican Sen. Cory Gardner is widely regarded as the most endangered Republican incumbent.

Next on the list of targets are three Republican senators from swing states: Arizona’s Martha McSally, North Carolina’s Thom Tillis and Maine’s Susan Collins. McSally and Tillis are both in their first terms, and neither appears to have instilled fervent loyalty in their state’s voters. Collins, of course, is a longtime favorite of Maine voters, having first won her seat in 1996, and may prove a more tenacious candidate.

Among the 12 currently Democratic-held seats on November’s ballot, only one — that of Alabama’s Doug Jones — is from a state that Trump carried in 2016, and is almost sure to carry again. Jones is in the Senate by virtue of a special election in which he had the great good fortune to have as his Republican opponent Roy Moore, the Bible-thumping friendship-cultivator of teenage girls.

As Jones cannot assume he’ll be that lucky this year (his Republican opponent will either be former Attorney General and Sen. Jeff Sessions or Auburn head football coach Tommy Tuberville), prudent Democrats should assume that they’ll need to pick up four Senate seats to get to 50. Should Biden or another Democrat win the presidency, that would give them the narrowest of majorities, relying on the Democratic vice president to be the Senate’s tie-breaking vote.

That that means for Democrats is they’ll have to pour resources chiefly into the four states — Colorado, Arizona, North Carolina and Maine — where Republican senators are most endangered. Their likely nominees — former Gov. John Hickenlooper in Colorado, former astronaut Mark Kelly in Arizona, Maine House Speaker Sara Gideon and former state Sen. Cal Cunningham in North Carolina, who became the Democrats’ nominee in the March 3 primary — stand either in their party’s mainstream or a little to the right of it, much like Biden himself. If voters are persuaded to cast their ballots for Biden, they will likely vote for these Senate candidates as well.

After those states, it’s down to tougher prospects. In Montana, the popular Democratic Gov. Steve Bullock recently announced his candidacy against Republican Sen. Steve Daines. In Kansas, should Republicans nominate the far-right Kris Kobach, who lost to a Democrat in the 2018 gubernatorial election, the Democratic nominee would certainly have a shot. And if polls in October show a Biden blowout looming, Democrats might even be able to pick up seats in such currently improbable locales as Iowa and Texas.

That said, how states will vote in Senate races depends more heavily than ever on how they vote for president.

Harold Meyerson is a contributing writer to Los Angeles Times Opinion section and editor at large of the American Prospect, a public policy magazine and daily web publication dedicated to American liberalism and progressivism.