Sen. Susan Collins of Maine, a prominent moderate Republican, raised campaign cash Monday for Pennsylvania GOP Senate nominee Pat Toomey, a conservative who as recently as last year was trying to defeat people such as Collins.

The endorsement could provide ammunition for Toomey against his Democratic opponent, Rep. Joe Sestak, as they battle to seize the middle ground and define each other as an extremist.

After a fund-raising lunch at the Union League, Collins and Toomey said they were united around the core Republican principles of lower taxes, limited government, and individual freedom.

Never mind that the Club for Growth, a free-market advocacy group that Toomey ran until he declared his Senate candidacy last year, had skewered Collins as "Comrade of the Month" for her vote for President Obama's stimulus legislation.

"This is a pivotal race," Collins said. "It is one of those key Senate races that is going to determine whether the Republicans are able to regain control of the Senate or at least increase our numbers so we can be an effective check on the excesses of this administration."

Toomey, who has made a concerted effort to reach out to moderate Republicans ahead of the general election, said that he believes in the same kind of "big tent" party that Collins does.

"As Republicans, there's lots of individual items we're going to disagree about, but there's a broad theme on which we agree," Toomey said.

Sestak's campaign said voters should not be fooled - Toomey is to the right of the Pennsylvania mainstream. It launched "Republicans for Sestak" at an event outside the Constitution Center, where four GOP voters from his Delaware County district praised him.

Toomey is an "ideologue," while Sestak is a "pragmatic, problem-solving" centrist, said Scott Jenkins, an investor and cochairman of the Republicans for Sestak steering committee. He supported Sestak in his first run for Congress, in 2006.

Toomey's campaign released the names of 40 Democrats from across the state who are supporting the former Lehigh Valley congressman, boasting that its cross-partisan group was 10 times bigger than Sestak's. Jonathon Dworkin, spokesman for Sestak, said 40 to 50 people were on the steering committee of Republicans for Sestak.

Tickets to the Collins-Toomey lunch cost $1,000 for individuals and $2,000 for representatives of PACs, according to an invitation to the event. The Toomey campaign declined to say how much was raised.

Al Schmidt, a moderate Republican who was the party's nominee for city controller last year and attended the lunch, said, "Events like this help the big tent get a little bigger."