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House Overview|By David Wasserman, December 11, 2015

To most Republican strategists, there's no bigger nightmare than Donald Trump as the GOP's presidential nominee in 2016. This week, just about every Democrat running for president, Senate, House, and their respective campaign committees sought to tie Republicans to Trump and brand them one big bunch of xenophobes. Talk of a down-ballot Republican apocalypse has reached fever pitch.

Even setting aside the remoteness of a scenario in which Trump would face Hillary Clinton in a one-on-one contest, such talk is premature and possibly overblown.

Given Trump's unpopularity with the electorate overall, there's a possibility he could end an era of very close and competitive presidential elections and suffer a landslide defeat (by modern standards). But what would that mean down-ballot? If Trump becomes his own radioactive island, GOP candidates in swing districts would have no choice but to renounce him and run far away for cover.

The challenge in assessing their odds for survival in such a scenario is that there hasn't been a blowout presidential election in a very long time. However, history is on the GOP's side.

Since 1960, there have only been three elections in which one candidate prevailed by a double-digit margin in a presidential race: Lyndon Johnson over Barry Goldwater in 1964 (by 22.6 percent), Richard Nixon over George McGovern in 1972 (by 23.2 percent), and Ronald Reagan over Walter Mondale in 1984 (by 18.2 percent). In all three instances, Democrats retained control of the House.

Despite the predictable outcome of each of the three landslides, there is scant evidence the losing side's demoralized voters stayed home in huge numbers or bolted their party en masse down-ballot compared to the previous presidential cycle. In each case, voters seemed to evaluate presidential candidates on a case-by-case basis but stuck with their core party preferences for Congress.

In comparing swings at the congressional level to swings at the presidential level during landslide years, swings in the national House vote tended to be much less dramatic. In 1972 and 1984, Democrats actually gained ground in the national House vote despite their nominee losing horrifically. That's good news for Republicans, who hope voters would distinguish their candidates from Trump.

1964 vs. 1960

Election Year Dem Presidential GOP Presidential Dem House GOP House

1972 vs. 1968

Election Year Dem Presidential GOP Presidential Dem House GOP House

1984 vs. 1980

Election Year Dem Presidential GOP Presidential Dem House GOP House

Source: U.S. House Clerk's Office

Of course, today we are in a totally different era of congressional elections than we were in the 1960s, 1970s, and 1980s. "All Politics is Local" has given way to a nationalization of House races in which there seems to be less opportunity than ever before for local candidates to distinguish their own brand from that of their party and their presidential nominee. 

In fact, the 1964, 1972 and 1984 elections had the highest percentage of split ticket voting in modern history. Back in 1964, 33 percent of American voters split their tickets (voting for the candidate of one party for President and another from the House), the highest we had seen since the turn of the 20th Century. In the 1972 Democratic washout, 44 percent of voters picked a different candidate for the House than the for the president. And, in the 1984 Reagan landslide, 43.7 percent supported a House member from a different party than the candidate they supported at the top of the ticket. Could we see that level of split ticket voting again? It's possible, but unlikely. 

The 2012 election saw the lowest percent of split ticket voting since 1908 at 5.7%. That's a welcome trend for Democrats: in a Trump scenario, they would have a field day putting Republican candidates on the defensive by forcing them to repudiate every one of Trump's statements, much as has been the case this week. Democrats would simply hope that the long-term rise in straight-ticket voting holds true, even if Trump loses in a landslide.

However, an even more forceful trend benefits Republicans: there are far fewer swing districts than there were 30, 40, or 50 years ago. Currently, we rate just 31 districts out of 435 seats as competitive. That means that even a sea change in presidential voting could produce only a small change in House seats, leaving Democrats short of the net 30 they would need to take back the House.

Cook Political Report National Editor Amy Walter contributed.