TALLAHASSEE, Fla. — Now that Republicans have made him the U.S. Senate front-runner, Marco Rubio is trying to weather potentially damaging revelations about his credit card use, double billing for airfare and murky finances.

A few months ago, the former Florida House speaker seemed an unlikely bet to beat Gov. Charlie Crist in the party’s Senate primary. And in those months, Rubio’s lavish spending has come under scrutiny of federal investigators.

In that same stretch, his poll numbers have soared, carrying him well ahead of Crist and forcing the once seemingly unbeatable governor to consider running as an independent for the Senate. Crist has until Friday to decide.

The federal investigation of Rubio’s spending habits comes as the Republican National Committee and its chairman, Michael Steele, draw ridicule for excessive expenditures, including a nearly $2,000 bill to entertain campaign donors at a Hollywood club featuring simulated bondage and lesbian sex.

Spending by Rubio and other GOP officials who cast themselves as fiscal conservatives opens them to charges of hypocrisy in a state hard hit by the recession. Florida’s unemployment rate was 12.3 percent in March, above the national average, and home foreclosures are widespread.

Republicans see the potential for significant gains in this fall’s congressional elections, with a legitimate shot of regaining control of the House and perhaps the Senate. But a family feud in Florida combined with questionable spending habits could thwart their political gains.

Questions have arisen about fundraising for and spending from two political committees Rubio formed as House speaker to benefit House candidates. The Miami Herald and St. Petersburg Times reported that little of the money raised went to candidates and some of it benefited Rubio’s relatives.

Much of the attention has focused on credit cards issued to Rubio and 27 other Republican leaders. The Internal Revenue Service is looking into the spending by Rubio and others Republican leaders, including a former state party chairman, Jim Greer, hand-picked by Crist.

Rubio charged nearly $110,000 to the party-issued card during his time as House speaker. Among the charged items were repairs to Rubio’s family minivan, grocery bills, plane tickets for his wife, and retail purchases, including one from a wine store.

Campaign aides have said the charges were legitimate; for instance, they said, the minivan had been damaged while being used on GOP business. They also said Rubio paid American Express $16,052.50 to cover nonparty expenses when they happened.

But Rubio recently sent the party a $2,417.80 check as reimbursement for personal airline flights he said were mistakenly charged to the GOP credit card. Rubio blamed the expense on an accounting error.

Rubio acknowledged that he made personal purchases on his party-issued credit card but said he paid those bills out of his own pocket. Other spending, he said in a statement, was “for legitimate political purposes”

Among the expenditures that have been called into question: a $134 charge at a men’s salon in Miami; hotel rooms for a family get-together; and purchases at a wine store.

So far, none of the reports has stalled Rubio’s campaign. He trailed Crist by 46 percentage points among Republican voters in the Quinnipiac University poll last year; this month, Rubio led Crist by 23 points in the same poll.

David Johnson, a Tallahassee-based Republican strategist, says internal GOP polling shows party voters know about Rubio’s spending issues, but they still support him. “They’re concerned, yep, ‘But we’re voting for him,”‘ Johnson said.