Many Democrats, popcorn in hand, are watching the Republican presidential primary with glee, believing that a Ted Cruz or Donald Trump nomination would be a disaster for Republicans up and down the ballot. That’s certainly possible — even plausible. But even if we assume that Cruz or Trump would get blown out in the general election — a shaky assumption — history suggests that it’s far from a guarantee that congressional Democrats would benefit.

Sometimes blowouts at the presidential level correspond with big swings in the makeup of the House of Representatives, and sometimes they don’t. Take a look at the table below, which shows the change in seat margin in the House in years with a presidential election since 1916 and the Democratic margin in the presidential race. (Republicans currently hold a 58-seat edge over the Democrats in the House.)

How the presidential vote affects the House DEMOCRATIC EDGE YEAR PRESIDENTIAL MARGIN CHANGE IN SEAT MARGIN 1916 +3 -44 1920 -26 -122 1924 -25 -46 1928 -17 -62 1932 +18 +198 1936 +24 +25 1940 +10 +12 1944 +8 +40 1948 +5 +150 1952 -11 -43 1956 -15 +4 1960 +0 -43 1964 +23 +73 1968 -1 -10 1972 -23 -25 1976 +2 +2 1980 -10 -69 1984 -18 -32 1988 -8 +4 1992 +6 -18 1996 +9 +7 2000 +1 +3 2004 -3 -5 2008 +7 +42 2012 +4 +16

There is a modest relationship between House and presidential results; about 50 percent of the variation in how the balance of the House changes is explained by the Democratic presidential vote percentage margin and the Democratic seat margin heading into the election. In other words: The presidential and House races are related, but how closely the two mirror one another varies widely depending on the election.

Remember when Democrat Lyndon Johnson blew out Barry Goldwater in 1964? The Democrats widened their House margin by 73 seats in that election, after coming into it with an 82-seat advantage.

As Goldwater was more than 50 years ago, Cruz is far more conservative than the average general election voter. But if he wins the GOP nomination, Republicans can take heart from the 1972 election, when Democratic nominee George McGovern, an uber-liberal, was pounded by Richard Nixon: The Democratic margin in the House shrunk by only 25 seats. That’s pretty amazing considering that Democrats held a huge majority in the House, giving the GOP a lot more room to pick up seats. This happened in part because many Southern Democrats voted for the Republican presidential candidate while also backing their Democratic representatives in the House.

The most recent presidential election decided by more than 10 percentage points featured only minor gains for the party that won the White House. In 1984, Republican Ronald Reagan, the incumbent, won by nearly 20 percentage points, but House Republicans narrowed the Democratic margin by only 32 seats. As in 1972, Republicans were unable to cut deep into the Democratic majority despite having a lot of possible seats to pick off.

There is one trend that could aid Democrats if Cruz or Trump were nominated: Fewer people cast split-tickets these days; instead, voters are increasingly choosing to back congressional and presidential candidates from the same party. So this could be a year in which a blowout at the presidential level filters down to the House.

But there’s a second trend that might counteract straight-ticket voting: Because of redistricting and the clustering of Democratic-leaning voters in big cities, there are many Republican districts that Democrats have little chance of winning — even if they win the national House vote (like they did in 2012). The median seat in Congress — according to presidential voting patterns — is a few percentage points more Republican-leaning than the country as a whole.

We just don’t know how nominating Cruz or Trump would affect down-ballot Republicans (Trump, in particular, doesn’t have any real precedent). It could end up being a complete disaster, like it was for the GOP in 1964. It could, however, mean very little.

Check out our latest forecasts for the 2016 presidential primaries.