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 Obama’s 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.