As the race between Hillary Clinton and Donald J. Trump teeters on a razor’s edge in Florida, their campaigns are scuffling over business that Mr. Trump may have conducted in Cuba while American companies were barred from operating there.

The issue has already started to percolate among the state’s large Cuban-American population and could have the potential to undo some of the progress that Mr. Trump — a pariah to many Hispanics — had been making there.

The revelations come as both sides are dumping considerable resources and time into the state, which is seen as critical to Mr. Trump’s hopes of winning the White House. Mr. Trump visited the Little Havana section of Miami this week. Mrs. Clinton just began running a Spanish-language television ad in Florida that attacks Mr. Trump on immigration and includes a 19-year-old Cuban-American saying that he will vote for her. In an attempt to further bolster Mrs. Clinton, her campaign announced on Friday that President Obama would visit Miami next week on her behalf.

The report that Mr. Trump’s corporation spent $68,000 on a 1998 trip to explore business opportunities in Cuba fit neatly into a line of attack the Clinton campaign has been pursuing recently. During the first presidential debate on Monday, and as she travels the country talking to voters, Mrs. Clinton has been arguing that, throughout his career, Mr. Trump has continually put his own interests above everyone else’s — whether the small contractors he has refused to pay or the taxes she says he still owes the federal government.