Harry Enten: “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.”
“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.”