The poll, recently conducted by Washington, D.C.-based Mason Dixon Polling & Research, asked 625 registered voters who they were likely to vote for if the primary was held today. Forty-eight percent responded that they would vote for Rubio; 37 percent favored Crist; and another 15 percent were undecided.
The poll has a margin of error of four percentage points.
The results are a sharp turnaround from last June, when a similar poll showed Crist leading with 51 percent. But researchers noted that the lead was based on name recognition. Crist's popularity has since declined significantly, while Rubio's name recognition and favorability rating has risen.
Ten months ago, only half of the GOP primary electorate had heard of Rubio, compared with 87 percent in March, according to the independent polling firm. Likewise, the percentage of voters with a favorable opinion of him has risen from 24 percent to 42 percent over the same time period.
Both Republican candidates hold double-digit leads against Democrat Rep. Kendrick Meek.
The campaign is becoming one of the closest watched races in the country.