I like Carson a lot. It’s nice to see him getting such strong support. I’m looking forward to some of the candidates shaking out and seeing where their supporters go.

According to a survey of likely Republican Iowa caucusgoers from Monmouth University, Ben Carson is tied with Mr. Trump in Iowa. The poll marks the first time in more than a month that the billionaire tycoon was not leading a poll in one of the first four states to hold nominating contests.

Mr. Trump and Mr. Carson were deadlocked with the support of 23 percent of the Iowans who were polled. Carly Fiorina, Senator Ted Cruz of Texas and Gov. Scott Walker of Wisconsin made up the next tier of candidates.

The poll shows the growing appetite for political outsiders in Iowa and could spell trouble for more traditional candidates who were counting on the state to propel them to the nomination. Mr. Walker has seen his prospects in Iowa dampen substantially in recent weeks and candidates such as former Gov. Mike Huckabee of Arkansas and former Senator Rick Santorum of Pennsylvania, who were once favorites in the state, are barely registering with voters.