Recent polling in the race had shown Nelson with about a 10-point lead over the Republican vying for his seat, although Quinnipiac’s polls have tended to show the race closer.

Democrats have been optimistic that a weak field on the GOP side has shored up Nelson’s reelection prospects and made it less likely that Nelson will face a serious threat. But with two polls showing the race within 5 points before the primary is even over, both parties are sure to make Florida a major focus of their efforts as they fight for narrow control of the Senate in 2013.

Former Rep. Dave Weldon (R-Fla.), who jumped into the race on Friday, was not included in either poll, but his name recognition is expected to pale in comparison to Mack’s at the start of his long-shot campaign.

The Quinnipiac poll of 1,722 Florida voters was conducted May 15-21 and had an overall margin of error of plus or minus 2.4 percentage points.

The NBC-Marist poll of 1,078 Florida voters was conducted May 17-20 and had a margin of error of plus or minus 3 percentage points.