Mack and his father, former Sen. Connie Mack III (R-Fla.), both endorsed and campaigned with Romney during the state's January primary, which the presumptive Republican nominee won handily.

But since then, Mack has been plagued by a series of pitfalls, including allegations from the LeMieux campaign that he primarily resides in California. Mack has also drawn fire for a mailer, sent for free under congressional franking privileges, that targeted residents outside his district.

Mack's campaign has insisted the letter was unintentionally mailed by an outside vendor, and released a letter from Rep. Aaron Schock (R-Ill.) noting that Mack had reimbursed the government for the mistake.

Neither Republican candidate has polled particularly well against the incumbent Nelson, and what had appeared to be a competitive Senate race now looks like an easy keep for Democrats.