A poll taken by the Mason-Dixon Polling & Research for the Tampa Tribune and WFLA-TV showed former housing Secretary Mel Martinez leads Martinez in the seven-man race 33 percent to 27 percent. But 18 percent of the respondents are still undecided.
A similar poll a month ago showed McCollum led 29-24 percent with 30 percent undecided.
The poll had a margin of error of 4.5 percent. It queried 500 voters Monday and Tuesday.
Matt Towery, chief executive of the polling company InsiderAdvantage, says it conducted a poll this week showing a virtual dead heat with McCollum slightly ahead. That result was well inside the margin of error of 5 percent.
Martinez is reportedly the choice of the White House in the Florida race for the seat vacated by retiring Sen. Bob Graham, D-Fla.
Businessman Doug Gallagher was third in both but trailed badly.