The mysterious billboards and digital ads surfaced in April, difficult for Miami’s drivers, internet surfers and social media users to miss. “Deporting immigrants is cruel,” one said, featuring the faces of Cuban American Republicans in Congress. More ads followed, most recently trying to denounce the politicians for a new state-run immigration detention center in the Florida Everglades known as “Alligator Alcatraz.”

Now, after months of anonymity, the leader of the unusual campaign, Michael B. Fernández, has decided to go public for the first time, explaining why he is spending millions — or even tens of millions — of his fortune on the ads.

Mr. Fernández, a billionaire philanthropist and the chairman of MBF Healthcare Partners, a private investment firm in Coral Gables, Fla., said in an interview with The New York Times on Friday that he hoped to “wake up the conscience” of Miamians, especially fellow Cuban Americans. He fears they fail to see parallels between the strongmen they fled and what he says is the United States’ eroding democracy.

“We are seeing a replay of what I saw when I was 12 years old and left Cuba,” said Mr. Fernández, 73, who is known as Mike. “It is beyond troubling. It is scary.”

Mr. Fernández is a former Republican who left the party more than a decade ago to register without party affiliation.

The ad campaign, run by a political group called Keep Them Honest, has made Mr. Fernández something of an outlier in Florida, which has moved decidedly to the political right. That trend has occurred throughout Miami-Dade County, where several cities have some of the country’s highest levels of foreign-born residents, most of them Hispanic. Republicans have defended President Trump’s crackdown on illegal immigration as necessary to ensure the rule of law after the number of migrants crossing the southern border surged in recent years.