Republican presidential hopeful Marco Rubio fired back Wednesday at rival Donald Trump during a campaign swing through New Hampshire, after the billionaire businessman targeted the Florida senator over his personal finances.

"[Trump] always gets weird when his poll numbers get a little down and goes off a little bit. I guess that's what's happening," Rubio told Fox News. "We are going to focus on our campaign -- and we will pray for Donald; that he finds some peace."

Rubio added: "I don't know what he is talking about. I don't have any debt in the world except a mortgage that I own with our family."

Trump first went after Rubio on his finances during a press conference Tuesday and kept up the criticism Wednesday, after becoming the first Republican to file for New Hampshire's first-in-the-nation primary ballot. Trump is at the top of most polls alongside Ben Carson, but Rubio has risen to third in many surveys.

"Marco Rubio has a disaster on his personal finances. He has a disaster on his credit cards," Trump said in the state capital of Concord.

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During his U.S. Senate run in 2010, Rubio faced similar criticism from then-opponent Charlie Crist, who attacked the upstart candidate on the same topic. At issue: whether Rubio as Florida House speaker crossed an ethical line by using his Republican Party of Florida American Express charge card for personal reasons. Rubio always maintained he repaid his personal expenses on the charge card in 2007 and 2008, totaling roughly $16,000. He also argued the card was a charge card -- payments had to be paid off every month, unlike a credit card where a balance is rolled over to the next billing period.

"It wasn't a credit card, it was an American Express charge card secured under my personal credit in conjunction with the party," Rubio  said on ABC's “Good Morning America” on Wednesday.

During the 2010 campaign, the Rubio team accused Crist -- a former Republican governor who left the party to run as an independent -- of leaking the credit card issue for political gain. Crist lost to Rubio in the election.