The St. Petersburg Times and Miami Herald, in a joint investigation, examined Rubio's use of an American Express credit card supplied by the Republican Party of Florida between 2005 and 2008. They found charges including $1,000 for repairs to Rubio's mini-van and charges at two supermarkets.
Rubio in a statement Wednesday said he was careful to repay the party for any personal expenses he charged.
"I was as diligent as possible to ensure the party did not pay for items that were unrelated to party business," Rubio told the newspapers in a written statement. "There was no formal process provided by the party regarding personal charges."
Katie Gordon, a Republican Party spokeswoman, said the policy was that the cards were supposed to be used only for party expenses.
Rubio has challenged Gov. Charlie Crist in the Republican primary for the U.S. Senate seat now held by George LeMieux, who was appointed by Crist to replace Mel Martinez. Rubio, far behind Crist in early polls, now appears to be running ahead of him.