The powerful anti-tax group the Club for Growth on Monday endorsed former state House Speaker Marco Rubio in Florida's Senate Republican primary, setting up a marquee showdown with national party leaders, who have already backed Gov. Charlie Crist.

The endorsement came just days after the group announced it would release a television ad attacking Crist for endorsing President Barack Obama¹s economic stimulus plan earlier this year.

Crist has the backing of by the National Republican Senatorial Committee. But Florida is an expansive state where campaigns have to buy ads in multiple markets, so the Club for Growth commitment could make the primary battle much more competitive.

The anti-tax group the Club for Growth said Thursday that it is ready to air a television commercial in Florida that hits Gov. Charlie Crist for endorsing the stimulus bill -- one day after the governor said in a CNN interview that he never backed the $787 billion spending legislation.

The ad uses video from a Crist appearance with President Barack Obama in February during which the governor talks about the importance of passing the stimulus package.

"Since Charlie Crist helped pass Barack Obama's spending program, nearly 200,000 Floridians have lost their jobs. Unemployment is the highest in decades. Personal income's down. And the deficit in Washington is three times larger," a narrator says in the ad. "Tell Gov. Crist to work on fixing Florida's economy, not passing more debt to our children."

When Pennsylvania Sen. Arlen Specter switched parties two months ago, he said he would issue contribution refunds "upon request."

Now the Club for Growth, the conservative political group that is a longtime Specter foe, wants to help any disgruntled Republican donors take now-Democrat Specter up on his offer.

The Club is asking the Federal Election Commission (FEC) if it can send a letter or make a telephone call to Specter's donors informing them that they can ask his campaign for a contribution refund.