Four House incumbents have already lost re-election bids so far this year, just a few months into primary season. And about a dozen more are facing legitimate threats in the coming months. Subscribe to read this story ad-free Get unlimited access to ad-free articles and exclusive content. Two Republican incumbents are squaring off for one district in California this week, the latest example of redistricting changing not just general elections but primaries as well, after two Democratic members lost in Texas last week. But the heaviest upcoming primary action will be on the Democratic side. A handful of Democratic incumbents in California will also face energetic challengers arguing that voters want a fresh face, part of a national slate of anti-incumbent candidates focused on generational change. A wealthy incumbent Democrat in Maryland is running against an even wealthier former congressman in a primary there. And other Democrats in states like Colorado and Massachusetts face challenges from their party’s left flank, part of a running battle between progressives and the Democratic establishment. The incumbent primary danger …