Support for Sen. Marco RubioMarco Antonio RubioAnother VPOTUS tries for POTUS: What does history tell us? Tensions swirl around Iran as administration to brief Congress Tensions swirl around Iran as administration to brief Congress MORE's (R-Fla.) potential presidential bid has slipped and Sen. Rand PaulRandal (Rand) Howard PaulRand Paul splits with Amash on Trump impeachment The Go-Go's rock the stage at annual 'We Write the Songs' DC concert GOP presses Trump to make a deal on spending 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.

ADVERTISEMENT

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 CruzRafael (Ted) Edward CruzJim Carrey fires back at 'Joe McCarthy wanna-be' Cruz Hillicon Valley: Google delays cutting off Huawei | GOP senators split over breaking up big tech | Report finds DNC lagging behind RNC on cybersecurity GOP senators split over antitrust remedies for big tech 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 Diane Rodham ClintonHarris readies a Phase 2 as she seeks to rejuvenate campaign Nevada Senate passes bill that would give Electoral College votes to winner of national popular vote 2020 Dems break political taboos by endorsing litmus tests MORE dominates her potential primary opponents, with 71 percent support to 12 percent for Vice President Biden and 5 percent for Sen. Elizabeth WarrenElizabeth Ann WarrenThe Hill's Morning Report - Trump says no legislation until Dems end probes Harris readies a Phase 2 as she seeks to rejuvenate campaign 2020 Dems put spotlight on disabilities issues 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.