Support for Sen. Marco RubioMarco RubioAt CPAC, Trump lashes out at media Conquering Trump returns to conservative summit Rubio brushes off demonstrator asking about town halls MORE's (R-Fla.) potential presidential bid has slipped and Sen. Rand PaulRand PaulConquering Trump returns to conservative summit Rand Paul rejects label of 'Trump's most loyal stooge' GOP healthcare plans push health savings account expansion 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 CruzBrietbart CEO reveals that Trump donors are part owners At CPAC, Trump lashes out at media Conquering Trump returns to conservative summit 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 ClintonReport: New national security adviser breaks with Trump on 'radical Islamic terrorism' EPA head previously used private email for government business Arkansas lawmaker proposes bill that would remove Clinton name from airport: report MORE dominates her potential primary opponents, with 71 percent support to 12 percent for Vice President Biden and 5 percent for Sen. Elizabeth WarrenElizabeth WarrenSanders and Schumer are right: Ellison for DNC chair Dean: Schumer's endorsement 'kiss of death' for Ellison How the candidates for DNC chair stack up ahead of Saturday's vote 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.