Support for Sen. Marco RubioMarco RubioSenators introduce new Iran sanctions Senate intel panel has not seen Nunes surveillance documents: lawmakers With no emerging leaders, no clear message, Democrats flounder MORE's (R-Fla.) potential presidential bid has slipped and Sen. Rand PaulRand PaulTrump, GOP fumble chance to govern GOP senators pitch alternatives after House pulls ObamaCare repeal bill Rand Paul takes victory lap on GOP health bill 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 CruzHow 'Big Pharma' stifles pharmaceutical innovation AIPAC must reach out to President Trump Under pressure, Dems hold back Gorsuch support 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 ClintonCorruption trial could roil NJ Senate race Infowars' Alex Jones apologizes for pushing 'Pizzagate' conspiracy theory Warren: 'Today is a great day... but I'm not doing a touchdown dance' MORE dominates her potential primary opponents, with 71 percent support to 12 percent for Vice President Biden and 5 percent for Sen. Elizabeth WarrenElizabeth WarrenInspector general reviewing HHS decision to halt ObamaCare ads Warren: 'Today is a great day... but I'm not doing a touchdown dance' The Hill’s Whip List: Where Dems stand on Trump’s Supreme Court nominee 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.