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.

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