Since the 1970s, the share of voters who split their tickets—supporting one party for president and the other in Senate races—has steadily declined. If that pattern persists in November, Republicans will likely lose their slim upper-chamber majority because the most competitive Senate races are clustered in blue or purple states where Trump faces the greatest resistance. To maintain control, Republicans will need either a dramatic Trump recovery in states such as Illinois and Wisconsin—or to convince more voters to split their ballots in Senate races than either side has typically persuaded lately. Neither will be easy.
In one respect, Democrats have helped Republican candidates to escape any Trump undertow. Although some individual Senate candidates have linked their opponents to the blustery nominee, Hillary Clinton has mostly chosen not to tie Trump to conservative thought but rather to define him as a fringe departure from it. Republicans are hopeful that will help conservative-leaning voters who can’t stomach Trump revert to their usual party loyalties in Senate races. “Voters are [receiving] multiple messages telling them that you can vote a different way down the ticket,” says Alex Lundry, co-founder of the GOP voter-targeting firm, Deep Root Analytics.
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.
Holding the Senate was always an uphill climb for Republicans this year because they are defending so many more seats than Democrats. But Donald Trump’s polarizing candidacy is converging with long-term shifts in voting patterns to make the hill even steeper.