At least three Senate victors could be seated immediately after the November elections, raising the possibility that Democrats could see their majority cut for the end of the year as Congress deals with several key pieces of legislation.
Lawmakers are typically seated in January. But deaths, a resignation and a series of Democrats taking jobs in the Obama administration forced six states to fill Senate vacancies through appointment since 2008, including those created by the president and vice president.
Terms for three of those appointed senators—from Illinois, West Virginia and Delaware—expire after elections Nov. 2.
State laws require replacements to be seated immediately, and Republicans are seen as having a shot at winning in Illinois and West Virginia. The GOP candidate in Delaware, tea-party-backed Christine O'Donnell, is trailing Democratic nominee Chris Coons by double digits in recent polls.
In Colorado, where the election is considered a toss-up, Republicans also intend to push for a speedy appointment.
The possibility of early seating has created a window for candidates such as Republican John Raese in West Virginia, who tells voters he could stop a last-ditch spending effort in the lame-duck session—the period between the election and installation of a new Congress. Mr. Raese is running for Senate against the state's Democratic governor, Joe Manchin.
"John is certainly going to Washington to oppose legislation," said Mr. Raese's campaign manager, Jim Dornan. Mr. Dornan said Mr. Raese would try to help pass legislation in the next Congress.
Rep. Mark Kirk, a Republican congressman running for President Barack Obama's old Senate seat in Illinois, has created a separate website for the issue, saveusfromthelameduck.com. He mentions it "pretty much everywhere he goes," his spokeswoman said.
Polls show him ahead of his Democratic opponent, state Treasurer Alexi Giannoulias.
"I would become the 42nd Republican senator, with the opportunity to put the breaks on any lame-duck overreach," Mr. Kirk said in a video on his website. Democrats are currently expected to lose about six to eight Senate seats.
The Senate's 41 Republicans can already block legislation at will, as Democrats need 60 votes to stop a filibuster. Pending business for the lame-duck session includes spending bills, the Dec. 31 expiration of the Bush tax cuts and the expected recommendations from Mr. Obama's debt commission, which reports in early December. Representatives for Majority Leader Harry Reid wouldn't comment on a lame-duck agenda.
If all three Republican candidates win in states that allow for immediate seating, Democrats would struggle to pass anything that would trigger a GOP filibuster. The party would need to wrangle as many as four Republican votes to proceed.
In July, a federal court in Illinois decided the two Senate candidates must compete in a separate special election, also on Nov. 2, to serve during the lame-duck session, which makes up the last weeks of Mr. Obama's uncompleted Senate term. Sen. Roland Burris, who was appointed as the president's successor in December 2008, has sued to be included on the special-election ballot. The U.S. Supreme Court declined to get involved in the case this week. Mr. Burris's lawyer is perusing the matter in U.S. appeals court.
Republicans want their candidate in Colorado, Ken Buck, to be seated immediately, should he defeat Democratic Sen. Michael Bennet. Mr. Bennet was originally tapped to replace then Sen. Ken Salazar, now Mr. Obama's secretary of the interior. A spokesman for Democratic state Secretary of State Bernie Buescher said that win or lose, tradition dictates Mr. Bennet will serve until January
The Senate normally deals with three to four appointments a year, said Senate associate historian Betty Koed. During the 79th Congress, a record 13 senators were appointed between 1945 and the 1946 midterms. Some stepped down immediately in accordance with state law, other resigned in December to give their successors a leg up on seniority. Others held on until January.