“Senator Snowe wants to focus on bringing down the deficit and getting the economy on track, and that’s where the priorities should be,” said Gov. Lincoln Chafee of Rhode Island, another moderate who served with Ms. Snowe in the Senate before leaving the Republican Party.

With the announcement by Ms. Snowe, the political center has all but given way in Congress, with both Republicans and Democrats who fashioned themselves as common-sense moderates stepping down or being booted out.

Abortion-rights groups say that only one Republican senator who strongly supports abortion rights, Susan Collins of Maine, will remain in 2013. Mr. Brown and Senator Mark Steven Kirk, Republican of Illinois, consider themselves pro-abortion rights, and Senator Lisa Murkowski, Republican of Alaska, has a mixed record on abortion issues.

Groups that seek to elect Republicans who favor abortion rights still exist, but they struggle.

“When you’re looking at the impact of all this, it should be of great concern to the United States of America,” said Senator Joe Manchin III of West Virginia, a conservative Democrat up for re-election.

Ben Nelson of Nebraska, the senator often considered the most conservative Democrat, and Ms. Snowe, seen as the most liberal Republican, will both be gone next year, as will Senator Joseph I. Lieberman of Connecticut, an independent who left a Democratic Party that would not tolerate his pro-Iraq war stand. They follow a parade of centrists out the Senate doors in recent years, including the Democrats Blanche Lincoln and Evan Bayh; a Republican-turned-Democrat, Arlen Specter; and two Republicans-turned-independents, James M. Jeffords and Mr. Chafee.

Without such dealmakers, it is anyone’s guess how major decisions on the tax code, budget deficit and entitlement changes will be made next year, regardless of the victors in the November election.

“I’ll be interested in how all this is resolved, but I’m going to be reading about it in the paper in a duck blind,” Mr. Nelson said.