Could a flailing Donald Trump campaign hurt down-ballot Republicans and cost them majorities in the Senate and House? That seems possible, if he loses to Hillary Clinton by a margin similar to that in most current polls and if voters keep on voting straight party tickets as they have increasingly in recent years.

The trend is startling. In 2012, only 4 percent of voters opted for Barack Obama and a Republican congressional candidate, and only 6 percent went for Mitt Romney and a Democrat. That 10 percent total was the lowest since the question was first asked in 1952.