Published: Apr 22, 2010 6:19 PM EDT

TALLAHASSEE, Fla. - Former Vice President Dick Cheney on Thursday endorsed Marco Rubio in Florida's GOP Senate primary, a high-profile endorsement announced in the midst of a federal investigation into spending on state Republican credit cards that involves Rubio and others.

Questions about spending haven't slowed Rubio's momentum as he's surpassed Gov. Charlie Crist in the polls. Cheney called Rubio a strong conservative leader.

"We can trust Marco to stand up to the Obama agenda that threatens our freedom, and promote clear conservative alternatives," Cheney said in a statement released by the Rubio campaign.

Cheney also implied Crist is guided more by polls than principles. "Charlie Crist has shown time and again that he cannot be trusted in Washington to take on the Obama agenda because on issue after issue he actually supports that agenda," Cheney said. "Lately it seems Charlie Crist cannot be trusted even to remain a Republican."

He urged Crist, who is considering running as an independent candidate, to either stay in the Republican primary or drop out of the race, saying a three-way race would benefit Democrats. U.S. Rep. Kendrick Meek is the Democratic front-runner.

"It's just another Washington politician telling Florida what to do. I don't think Floridians appreciate it. It doesn't matter," Crist said of Cheney's endorsement while speaking to reporters outside the Capitol. Meanwhile, a Republican Party of Florida lawyer wrote in a memo to party officials that if Crist runs as an independent, anyone who publicly supports him or gives him money would be removed from state and county Republican committees. "Any member who fails to formally revoke his or her public support and request the return of any contributions made to a candidate running against the candidate of the Republican Party would be in violation of RPOF rules," Jason Gonzalez wrote.

The GOP primary has been receiving national attention as the once-popular Crist has crashed in the polls and Rubio has risen from obscurity to become a favorite among conservatives. Over the past month, he's also picked up endorsements from former New York City Mayor Rudy Giuliani, former Massachusetts Gov. Mitt Romney and Florida House Speaker Larry Cretul. And after Crist outraised Rubio by about a 13-1 ratio during the first quarter of fundraising, Rubio took in $3.6 million in the last quarter compared with Crist's $1.1 million.

While momentum has swung, Crist still had $7 million in his campaign account at the end of last month, or almost double what Rubio had. A year ago, a Quinnipiac University poll showed Crist ahead of Rubio by 46 percentage points.

Last week, the same poll showed Rubio ahead by 23 percentage points. Cheney's endorsement comes as federal authorities are investigating spending on state GOP credit cards. Twenty-seven American Express cards were issued to party leaders and elected officials.

Rubio, a former House speaker, had a party card for nearly four years and admits using it for some personal expenses, but says he personally paid American Express for non-party charges.

Gov. Crist cancels appearance at Charlotte sports park

In other news concerning Gov. Crist, it has been announced that the governor has canceled his scheduled appearance at this upcoming weekend's Stone Crabs game against Palm Beach.

No reason was given for the cancelation.

Related:

Crist announces run as independent candidate

Marco Rubio comments on 2010 U.S. senate election