Rasmussen reports that Charlie Crist trails Marco Rubio by seven points in a three-way race. In March, Rubio was up by 17 points, so it seems Crist's veto of the GOP education bill and his TV ads have helped him.
Update: Nate Silver has a good analysis:
If you play around with the "what-if calculator" that Pollster.com's Mark Blumenthal created, and give Rubio, say, 75-80 percent of the Republican vote along with a bare minimum of 25 percent of the independent vote and a few percentage points of Democratic support, he seems to have a floor of about 36 percent of the vote overall, whatever reasonable assumptions you might make about turnout. Indeed, given the mood of the electorate, I would be very surprised if Rubio wound up with less than 36 or 37 percent of the vote, unless there were some major scandal (always possible) or he ran a terrible campaign (also possible).
In a three-way race, what this means is that Rubio is guaranteed at least second place. He could still lose if the race were Crist 37, Rubio 36, Meek 27, or Meek 37, Rubio 36, Crist 27, but you can't get more than one-third of the vote in a three-way race and do worse than second.
This is advantageous, since it means that nobody much will defect from you for strategic reasons. Say, for instance, that your order of candidate preference were Rubio, Crist, Meek. If the polling at some point in late October were Meek 40, Crist 35, Rubio 25, a voter might consider defecting from Rubio to Crist to prevent Meek from being elected, thinking that his vote would otherwise be wasted. But this doesn't come into play if it's Meek 40, Rubio 35, Crist 25 -- then switching to Crist only helps to ensure Meek's election. Nor does it come into play if the polling is Crist 40, Rubio 35, Meek 25 -- then Meek isn't really in the running and you'll simply vote your first prefernce (Rubio) over your second (Crist).
By the same logic, third place is a dangerous place to be in a three-way race: then you can get into a vicious cycle where voters ditch you because they're afraid of wasting their vote, which in turn makes your problems worse and worse. This is precisely why support for third-party candidates often does tend to collapse at the end of an election if the candidate looks as though he's not going to be able to get over the hump.
I wonder: if Crist is in third place at the end of the campaign, would he pull a Dede Scozzafava and endorse the Democrat out of spite? If Democrat Kendrick Meek is in third place, would he endorse Crist if the governor agreed to caucus with Senate Democrats?