Support for Sen. Marco RubioMarco RubioSenate clears water bill with Flint aid, drought relief What Trump's Cabinet picks reveal House passes water bill with Flint aid, drought relief MORE's (R-Fla.) potential presidential bid has slipped and Sen. Rand PaulRand PaulTrump stumps for Louisiana Senate candidate ahead of runoff Giuliani won't serve in Trump administration Will justice in America be Trumped? MORE (R-Ky.) leads in the key early-voting state of Iowa, according to a new GOP primary poll conducted by the Democratic firm Public Policy Polling.

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Rubio's support has slipped to 11 percent, down from the 16 percent support he had when PPP last polled the state in February. The result come as Rubio fends off conservative criticism for his strong support of a bipartisan immigration bill in the Senate. 

Paul, meanwhile, has risen from 10 percent support in February to 18 percent today.

The Florida senator's dip could be driven more by others' rise than his own stature, however. 

Rubio's favorable rating stayed unchanged at 54 percent, with a statistically insignificant change from 13 to 14 percent in those who view him unfavorably.

The possible candidates are all very close to one another, however, well within the poll's 6.2 percent margin of error. 

Paul is just 2 percentage points ahead of New Jersey Gov. Chris Christie (R), who has 16 percent support. 

Former Florida Gov. Jeb Bush (R) pulls 14 percent, Sen. Ted CruzTed CruzSenate passes stopgap funding bill, averting shutdown Senate advances funding measure, avoiding shutdown Cruz, DeSantis to introduce constitutional amendment on term limits MORE (R-Texas) is nipping at Rubio's heels at 10 percent support and former Sen. Rick Santorum (R-Pa.), who won Iowa's 2012 presidential caucuses, sits at 6 percent.

This isn't the only recent PPP poll showing Rubio trailing the field of potential candidates, after some polls earlier this year showed him as the front-runner. 

PPP's recent poll of Texas Republicans found Rubio in sixth place among candidates, with 6 percent support, and a recent poll of Montana Republicans found him in fifth place.

On the Democratic side, Hillary ClintonHillary Rodham ClintonSecret CIA assessment: Russia was attempting to assist Trump Joy Behar: Why do I have to be nice about Trump? Poll: Republicans think media ‘intentionally misled the public’ about polling MORE dominates her potential primary opponents, with 71 percent support to 12 percent for Vice President Biden and 5 percent for Sen. Elizabeth WarrenElizabeth WarrenDemocrats: Where the hell are You? Dodd-Frank ripe for reform, not repeal Senate Dems offer bill to curb tax break for Trump nominees MORE (D-Mass.). In the general election, Clinton leads all comers by 7 percentage points or more.

PPP's automated poll of 668 voters, including 250 GOP primary voters and 260 Democratic primary voters, was conducted from July 5-7. 

The margin of error for the total poll was 3.8 percent, with margins of error of 6.1 percent and 6.2 percent for the Democratic and Republican primary samples.