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Rubio has maintained an aggressive fundraising pace despite frustration among some Republicans, banking more than $3 million over the past three months. That was the best haul ever for his political action committee, according to an official with his campaign; Rubio has already raised $5.28 million in 2013.

But a Rasmussen poll released last month showed Rubio's favorability had dropped 15 points among Republicans since February.

A Public Policy Polling survey of Iowa, the first state to vote in the Republican nomination fight, found Rubio earning the support of 10 percent of voters, good for fourth place behind Sen. Rand PaulRand PaulWe can put America first by preventing public health disasters Conservative activists want action from Trump McConnell: 'Big challenge' to pass ObamaCare repeal in Senate MORE (R-Ky.), New Jersey Gov. Chris Christie and former Florida Gov. Jeb Bush.