One footnote to Donald Trump's upset victory that's worth pointing out.

The Republicans had an uphill battle to keep the Senate in 2016, given that they would be defending a number of seats in blue-leaning states. As Hillary Clinton's candidacy gained strength at various points over the summer and fall, it seemed even more likely that the Senate would flip back from red to blue, two years after it went the other direction.

Trump's strength, though, meant that this didn't happen. At one point, it looked like Republicans might need to rely on crossover voting, people who would skip the presidential race or vote for Clinton but then vote for the Republican Senate candidate. In the end, that didn't happen either. Trump still received a lower percentage of the vote than most Republican senators, but his victories in these states — he won all but four — meant that Republican Senate candidates could rely on support from their base.

There do appear to have been two races in which crossover ballots came into play (according to these still-preliminary returns). In Missouri, there was some portion of the electorate that voted for Trump as well as the Democratic candidate in the state's Senate contest. Sen. Roy Blunt (R-Mo.) was expected to face a tough contest against Democratic challenger Jason Kander, who ran an effective campaign as a centrist Democrat. It's possible that Kander's support percentage overlaps with Trump's because some people voted in one race and not the other, but it's not likely. There was a Trump-Kander vote, but it didn't hurt Blunt.

In Ohio, there was an opposite effect: A Clinton-Rob Portman vote. This is perhaps less surprising, given that Ohio's Republican establishment — led by Gov. John Kasich — was constantly struggling against Trump's political position.

In the end, this was the sort of crossover vote that Republicans thought they'd need. Portman didn't.

The four Republican Senate candidates who are losing as I write are also the four in states where Trump is also losing. Only in Nevada did Trump outperform the Senate candidate. That's the link that Republicans didn't always expect to see and are happy to take: voters, heavily Republican voters, voting for the Republican candidates.