The districts seeing the most spending show why House Democrats are in trouble

The battle for control of the House has been spreading into bluer and bluer turf as Election Day creeps nearer, with more than 60 percent of ad spending the past two months flowing into districts President Joe Biden won by 5 or more points just two years ago.

The spending landscape illustrates the challenging pattern for Democrats and their slim majority: Many of their most vulnerable seats have already slid off the edge of the map, and Republicans are pushing them back even further, forcing an expensive defense of many...