Those results didn't make much of a difference two years later. Thirty-five states held Senate elections in 1990, and in 22 of them, the same party that won the presidential race also won the Senate seat. But in 13 states, it flipped; in 12 states, the state voted for a Democrat after two years earlier voting for the Republican. In 1992, 11 of 34 states split their vote -- backing the Republican in the presidential race and the Democrat in the Senate one, or vice versa. What's more, the margins of victory weren't really linked. In some places, Bill Clinton barely won while the Democratic Senate candidate romped.
We can plot that on a graph. The closer a dot (representing a Senate race) is to the diagonal line, the closer the state was to preferring a party's presidential candidate and Senate candidate to about the same extent. In 1992, the Senate races are just a blob.
Now compare that to the Senate races in 2014, compared to the results from the 2012 presidential race.
In the past few cycles, we've seen more and more that Senate results mirror presidential results.
In 1990, 63 percent of the states that held Senate races voted for a candidate from the same party as their presidential pick two years earlier. In 2014, 91 percent of states did that.
We looked at this phenomenon after the 2014 elections in regard to House district results in the presidential race. House races still end up being uncontested a surprising amount of the time, but you can clearly see the same clustering around that diagonal line in the middle.
This has occurred as the polarization of Congress has increased, a complex shift that's intertwined with a number of other factors. The trend above is why we might assume that the presidential election this year will rely heavily on partisan support for each candidate -- particularly given how negatively each candidate is viewed by the other party.
Donald Trump's argument for winning in November is that he can break the partisan hold enjoyed by recent winning presidential candidates. Maybe. The concern from Republicans hoping to win or hold seats in November is just the opposite: That he'll drive voters to vote for Democrats, prompting some Republicans to try and figure out how they might distance themselves from the top of the ticket -- to, in other words, hope for the return of split-ticket voting.
If that does happen, it would break with recent tradition. And it would make Donald Trump, against all odds, look something like a top-of-ticket candidate in the mold of George H.W. Bush -- which would frustrate no one in the world more than George H.W. Bush.