It is more expensive for a House incumbent to lose: the 32 ousted representatives spent an average of $3.1 million trying to keep their seats, while their challengers spent $2.5 million.

All those figures represent money spent by the candidates themselves, without factoring in the ever-rising flood of funds spent by the political parties and independent groups like “super PACs.” And the swing-state incumbents are getting plenty of fund-raising help, Mr. Mann said.