Bloomberg reported that Carson, a retired neurosurgeon, has 28% support from those potentially participating in Iowa’s upcoming caucus. Trump, meanwhile, has 19% of the likely participants. Carson has seen a 10% surge in his polling figures since August, while Trump’s numbers have declined by 4%.

“His standing has improved in every way pollsters traditionally measure,” said J. Ann Selzer, the president of West Des Moines-based Selzer & Co., the firm in charge of conducting the poll, to Bloomberg. “This might be a wake-up for Donald Trump.”

Jeb Bush, meanwhile, has just 5% support.

The poll was conducted from Oct. 16-19 and surveyed 401 “likely” Republican caucus participants.

On Thursday, Trump stirred controversy with a retweet that slammed Carson for his lead in Iowa. Trump later deleted the tweet, saying an intern sent it out.