At the start of the campaign, Democrats looking to regain control of the Senate viewed Mr. Santorum as one of the most vulnerable Republicans.

The failures of urban liberalism weaken the entire party as it gets ready to take on the increasingly vulnerable Republicans.

Democrats view Ms. Capito as one of the most vulnerable Republicans in the House.

Senator Abraham had long been viewed as one of the most vulnerable Republicans in the Senate.

His priority is to retain control of the House, and there are plenty of vulnerable Republicans holding traditionally Democratic seats.

Democrats who were expected to win easily are hard-pressed, and supposedly vulnerable Republicans are doing well.

One reason is that four vulnerable Republicans were defeated in the last election, meaning that the lowest-hanging fruit has already been picked.

"These are the most vulnerable Republicans," said Dan Cantor, the executive director of the Working Families Party, one of the groups behind the campaign.

National Journal's Cook Political Report named Schilling one of the top 10 Republicans most vulnerable to redistricting in 2012.

And a last-minute advertising blitz by the Republican Party probably shored up several vulnerable Republicans.