Responses to the “generic ballot” poll question suggest a partisan electoral wave is building. But the fight for control of the House isn’t a single national election. It will be fought district by district, and national Democrats face challenges on the ground even with the generic ballot favoring them.

In Michigan, according to America Votes 2007-2008, the statewide congressional vote shifted noticeably from 2004 to 2006 — from 49 percent Republican and 48 percent Democratic to 53 percent Democratic and 44 percent Republican — but that popular vote surge for the Democrats didn’t translate to a shift of even a single House seat.

GOP strategists had done a good job drawing district lines to protect Republicans. Most political junkies look to past election results to see which incumbents and districts could be at risk in the next election. This year, the focus has been on 23 House Republicans sitting in districts carried by Hillary Clinton in 2016.

That’s certainly a good place to start, but it’s a bad place to stop, especially in an electoral wave. Where else should you look for vulnerable House Republicans?

Dramatic reversals

Incumbents who win with nearly 60 percent are often categorized as “safe,” but a quick glance at recent election results finds that Republicans who won by 20 points in 2004 weren’t slam dunks for re-election two years later during a Democratic electorate wave, especially if those House members had not faced a serious test in years.