If the Trump collapse and Clinton surge continue, they could reveal the perils of partisan redistricting. That strategy created so many marginal Republican districts that if the GOP loses the bulk of the seats at or below R+2, it would also lose its congressional majority. A catastrophe that claimed every GOP seat at or below R+4 would bring the GOP caucus close to the size of today’s House Democrats.

More than that, many seats the Republicans lost would belong to newcomers, who include the most vocal Tea Party conservatives. Once again, this is an indirect result of gerrymandering, which typically ensures safer seats for the most senior party members.

In Pennsylvania, for example, three of the four marginal GOP districts are held by incumbents with less than five years of Congressional experience. By contrast, only one of the six most senior members of the Republican delegation has a seat with a PVI rating below R+6.

Of course, Democrats shouldn’t be over-confident, even amidst a decisive Trump defeat. The influence of a presidential contest weakens as one travels down the ballot, and incumbents typically insulate themselves well enough to earn reelection rates north of 95 percent.