But those messages are competing against a long-term evolution in voters’ behavior that has made it tougher for all senators to survive in effect behind enemy lines—in states that usually prefer the other party for president. As party-line voting inside Congress has reached near-parliamentary levels, voters have responded by treating congressional elections less as a choice between individuals and more as a parliamentary-style referendum on which side they prefer to control the majority.

That’s a big change from even the 1970s and 1980s, when conservative Democratic senators still dominated Southern states stampeding toward the GOP in presidential elections, just as moderate Republican senators thrived in coastal states trending toward the Democrats. In the modern apex of cross-pressured voting, fully 28 percent of voters supported one party’s presidential nominee and the other’s Senate candidate during Richard Nixon’s 1972 landslide, according to data from the National Election Studies analyzed by the Emory University political scientist Alan Abramowitz.

Since then, as Abramowitz has calculated, the share of voters who back one party for president and the other for Senate has fallen relentlessly. It declined to 23 percent on average during the 1980s, 16 percent across Bill Clinton’s two races, 13 percent in 2008 and just 10 percent in 2012.

With fewer voters dividing their loyalties, Senate results have increasingly followed the presidential map. After the reelection victories of both Richard Nixon (in 1972) and Ronald Reagan (in 1984), Republicans controlled only about half the Senate seats in the states that voted for each man twice. After President Obama’s reelection, Democrats held over four-fifths of the Senate seats in the 26 states that backed him twice (although the party lost some of those seats in 2014).

That history is especially relevant this year because Republicans are defending seven (of the 11 total) Senate seats they still hold in the two-time Obama states. Of those, Clinton is solidly leading in Illinois, Wisconsin, Pennsylvania, and New Hampshire; the Democratic Senate nominees also lead in those states. The presidential race is much closer in Iowa, Ohio, and Florida—where the Democrats’ Senate prospects are also cloudier.

The Senate-Presidential Connection

The final margin between Clinton and Trump will shape all of these contests. In 2012, exit polls showed that about 85 percent or more of both Obama and Mitt Romney voters also supported their party’s Senate candidates in almost every competitive state. Recent public polling shows that at least 82 percent of Clinton and Trump supporters are also backing their party’s Senate candidate in Wisconsin, Pennsylvania, New Hampshire, and Florida, as well as in North Carolina which Obama won once. (Those numbers are likely to rise by Election Day because they still include undecided voters.) If Trump slips further, these dynamics suggest he could endanger GOP senators in Arizona and Missouri.