But Club for Growth executive director David Keating said Tuesday that the survey also showed that Rubio has room for growth while Crist’s support is “softer than what a lot of people are assuming.”

“It’s clearly a winnable race, it’s not going to be an easy race. ... Rubio’s going to have to be able to communicate his message and his position on issues,” Keating said.

The poll showed Crist with 51 percent to Rubio’s 21 percent in the initial head-to-head ballot test. The survey of 500 likely primary voters was conducted by Basswood Research for the club on June 13 and 14 and had a 4-point margin of error.

Those findings closely mirror the results of a Mason-Dixon poll conducted June 24-26, which found Crist ahead by 28 points.

The poll conducted for the club went on to test several messages through informed ballot questions that focused mainly on Rubio’s biography, his connections to former Gov. Jeb Bush (R), and his fiscal and social conservative views.

“The shocking part is not so much Rubio going up but Crist going down so dramatically” when the informed ballot focused on Rubio without any negative information about Crist, Keating said.

Other parts of the informed ballot tested primary voters’ opinions on federal spending and President Barack Obama’s stimulus plan, which Crist publicly supported.

Rubio needs to build his statewide name identification, and he would no doubt benefit from the deep pockets of the Club for Growth Political Action Committee, should the group decide to back him and air TV ads to boost his campaign.

While Keating said Rubio “is clearly a very exciting candidate and would be a terrific Senator,” he said the club has not yet made any decision about who it would endorse and has no time frame for when that decision might come. He did acknowledge that the club generally gets involved in races earlier rather than later.

Rubio’s campaign is set to make a final fundraising pitch on a conference call with Sen. Jim DeMint (R-S.C.) Tuesday afternoon ahead of the Federal Election Commission’s second-quarter filing deadline. His campaign is more than happy to push an anti-tax message that is music to the ears of a group like the Club for Growth.

“We believe the American taxpayer will find a friend in Marco Rubio, who will lower their taxes and ensure their tax dollars aren’t wasted away in Washington,” Rubio spokesman Alex Burgos said Tuesday afternoon.