In 1964, President Johnson beat Barry Goldwater by 22 points, 61 to 39 percent, carrying 44 states, according to Vital Statistics on Congress, but while Democrats did pick up 37 seats in the House, their gain in the Senate was a single seat. In 1972, when President Nixon was pounding George McGovern by 23 points, 61 to 38 percent, and carrying 49 states, Republicans gained only 12 House seats and actually had a net loss of two seats in the Senate. For President Reagan’s “lonely landslide” in 1984—when he beat Walter Mondale by 18 points, 59 to 41 percent, and carried 49 states—his party gained only 14 House seats and again lost two Senate seats. The counter to those examples is that people are doing more straight-ticket voting and less ticket-splitting now than in those days.
Two factors make me suspect that even if Clinton wins by a much larger margin than, say, Obama’s win over Romney four years ago, I don’t think the down-ballot implications would be that huge. In the House, there are fewer competitive districts than at any point in our lifetimes; between natural population sorting and gerrymandering, there just isn’t much elasticity in the House these days. In the Senate, the GOP majority is absolutely on the line; my guess it will end up 50-50, give or take a seat or two, but given voters’ doubts about Clinton, the “don’t give Hillary Clinton a blank check” argument may well be a politically potent one, and a lot of hold-their-noses Hillary voters may well look for a check and balance down-ballot.
The primary fear for Republican strategists is that non-Trumpeteer Republicans might just opt to stay home, making other GOP candidates collateral damage. My hunch, though, is that voter turnout might well be fairly normal in this election. Sure, some may stay home, unenthused over Clinton and/or Trump, but I think there will be a lot of people turning out simply to vote against one or the other.