There is now a clear warning sign for Donald Trump in a key early state

Donald Trump is still leading a new poll of Iowa by a comfortable margin.

But the Republican field begins thinning, ever so slightly, the poll provides a warning sign for Trump.

The poll, from Public Policy Polling and released on Tuesday, shows that Trump is still the front-runner in Iowa, the crucial first-caucus state. Trump captures 24% support among likely Republican voters in the state, compared to 17% who support retired neurosurgeon Ben Carson. Former Hewlett-Packard CEO Carly Fiorina came in third place with 13% in the poll.

But the survey also shows that Trump is significantly weaker in a key metric: head-to-head matchups.

Trump would lose to Carson in a hypothetical matchup, garnering only 33% to Carson’s 60%. Trump would also suffer defeats in the Hawkeye State to Fiorina, and Sen. Marco Rubio (R-Florida). The only head-to-head matchup tested in which he would win came against former Florida Gov. Jeb Bush (R).

Trump, however, has performed well in head-to-head matchups in previous polls nationally. According to a Monmouth survey of likely Republican voters nationwide released earlier this month, Trump only lost to Carson in a theoretical head-to-head collision.

It’s far too soon to tell if the PPP numbers indicate legitimate problems for Trump — as many observers note, Trump has defied conventional expectations at every turn. And as PPP reports, Trump is still actually polling better than he was after the first GOP debate, capturing higher favorability ratings among Iowa voters as well.

Still, the poll hints that he could face more trouble as the field thins from its current level of 15 candidates. Wisconsin Gov. Scott Walker (R), who exited the field in stunning fashion Monday, urged more candidates to withdraw while taking not-so-subtle shots at Trump.

“As more people drop out there may be a closing of the field around Trump’s remaining foes,” PPP’s Tom Jensen wrote along with the poll.

And the real-estate mogul has also seen a slight dip nationally following last week’s Republican presidential debate. According to CNN, Trump leads the pack with 24% of the Republican vote, a dip of 8 points from another CNN poll released earlier in the month. Meanwhile, Fiorina and Rubio have gained significantly.

Fiorina’s rise — which many attribute to a strong performance at the second Republican debate — has seemingly spooked Trump. The reality television star has slammed Fiorina repeatedly since the debate, attacking her controversial record at HP.

But Trump’s comments and attacks on other candidates so far haven’t been a hit with Iowa voters.

PPP reported that the vast majority of likely Republican primary voters in Iowa (59%) disapproved of the real-estate mogul’s remarks on Fiorina’s face, which he later insisted meant to refer to her “persona.” Almost as many (49%) disapproved of Trump’s comments about former Florida Gov. Jeb Bush’s (R) wife Columba Bush.

“For all the controversy over the last six weeks,” though, said Dean Debnam, the president of PPP, “Donald Trump’s in a slightly stronger position now than he was after the first debate.”

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