Overriding those changes, though, is a thinning of pragmatic, centrist veterans in both parties. Among those leaving are some of the Senate's most pragmatic lawmakers, nearly half the House's centrist Blue Dog Democrats and several moderate House Republicans.

That could leave the parties more polarized even as President Barack Obama and congressional leaders talk up the cooperation needed to tackle complex, vexing problems such as curbing deficits, revamping tax laws and culling savings from Medicare and other costly, popular programs.

"This movement away from the center, at a time when issues have to be resolved from the middle, makes it much more difficult to find solutions to major problems," said William Hoagland, senior vice president of the Bipartisan Policy Center, a private group advocating compromise.