Mack had decided against vying for the Senate in the spring when two of his former colleagues in the Florida House, Adam Hasner and Mike Haridopolos, were running, The Miami Herald reported.
But Haridopolos dropped out of the race in the summer, and none of the other three candidates has emerged as a clear front-runner.
The Herald noted polls have shown Sen. Bill Nelson, a Democrat, would beat any of the three Republican candidates -- former U.S. Sen. George LeMieux, former gubernatorial candidate Mike McCalister and former Ruths Chris steakhouse chief Craig Miller. Nelson also raised $2 million last quarter, much more than the Republican candidates, but he could be hurt by President Barack Obama's poor poll numbers.
The Mack adviser, David James, said the Republican representing Fort Myers is assembling a team and would announce more about his candidacy in the coming weeks.
Mack's name is well-known: His father was a U.S. senator from Florida and his wife, Mary Bono Mack, is a member of Congress who had been married to Sonny Bono, the late singer who became a congressman before his death.