Norquist's grassroots organization expressed deep disappointment that Crist "went against his commitment to Floridians and broke his pledge as governor to oppose and veto any and all tax increases."

"The news was particularly surprising," since Crist recently "signed the federal Taxpayer Protection Pledge in his race for the U.S. Senate," ATR said in a statement.

Right now, Crist remains the odds-on favorite to keep the open Senate seat in the GOP's column. But if the Club for Growth, flush with cash, gets into the race, that could help Rubio with both money and message. Club membership "has never been higher," and the grassroots group has earned notoriety for winning upsets in party primaries, though not always with good results in the end.

For example, the Club last year opposed incumbent Republican Rep. Wayne T. Gilchrest on Maryland's Eastern Shore in his primary. He lost to conservative Andy Harris, who was defeated by Democrat Frank Kratovil in the general election.

Moreover, most of the Club's victories have been in congressional districts where it's cheaper and easier to have an impact in lower-turnout primary races. But party strategists question just how effective the Club could be in one of the costliest media markets in the country and whether they would be able to spend the kind of big money needed to change the direction of the race.

All of this has made life more complicated for Sen. John Cornyn of Texas, the National Republican Senatorial Committee's chairman who recruited Crist.

To be sure, the governor is a proven vote-getter who is seen as the only candidate capable of winning in a state that has become a lot more competitive in recent elections.

But taxes and spending have become the GOP's strategic issues in its bid to repair a damaged brand, and the party's restive base is in no mood to vote for anyone seen as going soft on either one of them.