Rubio banks $3 million in second quarter

That's an improvement of some $700,000 over the first quarter of 2012, and brings his annual haul up to $5.28 million. Rubio operates a joint fundraising committee that splits donations between his official campaign and a leadership political action committee, and now counts some $3 million cash on hand.

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The solid fundraising numbers come despite some consternation on the right over Rubio's push for a comprehensive immigration reform bill. A Rasmussen poll released last month showed Rubio's favorability had dropped 15 points among Republicans since February.

But the Florida lawmaker is still seen as one of the party's most likely presidential candidates in 2016, when he is also facing a reelection battle for his Senate seat.