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.