The Senate has provided Mr. Rubio with a prestigious platform, to write books, travel the world, deliver speeches and, today, mount a run for the White House. But he has told friends that the job has imposed its own form of financial hardship, and he expressed occasional envy of colleagues in the private sector.

Mr. Rubio’s Senate salary of $174,000 is far less than he earned as a lawyer and consultant, and the Rubio family expenses are significant. All four of their children attend parochial school.

He has looked for other ways to bring in cash: Shortly before running for the Senate, Mr. Rubio agreed to teach at Florida International University, for $69,000 a year. (The salary was later reduced.) Those involved in the negotiations said it was clear that Mr. Rubio’s finances were stretched.

“I think that was an issue,” said Steve Saul, who was vice president for government relations at the university when Mr. Rubio was hired. “He needed the money.”

The Rubios have further supplemented their income with royalties from his two books and Mrs. Rubio’s work for Mr. Braman, Mr. Rubio’s wealthy campaign donor.

But Mrs. Rubio’s firm, JDR Events, has had its own bookkeeping lapses. Over the past few years, she failed to pay annual business licensing fees to the City of West Miami, despite nine written notices and repeated phone calls to her home, records show.

After The Times made an inquiry with the city on May 26, a check arrived from Mrs. Rubio two days later for the $637.50 she still owed. In a handwritten note to the city, she said she had mistakenly believed her payments were up to date.

“My apologies,” she wrote.