A Quinnipiac University poll released Tuesday shows that Trump is the Republican presidential front-runner in the early voting state, with 25% support among GOP voters likely to participate in Iowa's February 1 caucuses.
But Cruz, the Texas senator, has moved into second place with 23% support -- double the backing he had just four weeks ago. Carson has fallen to third at 18% support, down 10 percentage points from his first-place perch a month ago when he led Trump, 28% to 20%.
Florida Sen. Marco Rubio is in fourth place with 13% support -- the only other candidate in double digits in Tuesday's poll.
Former Florida Gov. Jeb Bush and Kentucky Sen. Rand Paul are tied for fifth place at 5% each and Carly Fiorina at 3%. No other candidate tops 2%.
"Last month, we said it was Dr. Ben Carson's turn in the spotlight. Today, the spotlight turns to Sen. Ted Cruz of Texas. The Iowa Republican Caucus has become a two-tiered contest: Businessman Donald Trump and neurosurgeon Ben Carson lead on the outsider track, and Sens. Ted Cruz and Marco Rubio lead among party insiders," said Peter A. Brown, assistant director of the Quinnipiac University poll, in a release announcing the results.
"The other candidates will need miraculous comebacks to crack the top tier with slightly more than two months before the voting begins," Brown said.
While Carson has seen his poll numbers drop, his favorability rating -- at 79% positive to 15% negative -- is still the best margin in the GOP field.
He's followed by Cruz (73% view him favorably while 15% see him unfavorably), Rubio (70% to 18%) and Trump (59% to 34%). Bush's rating -- he's seen favorably by 39% and unfavorably by 53% of likely Iowa Republican voters -- is underwater.
The pollsters found that 24% of likely Iowa GOP voters say the economy and jobs is the most important issue in deciding who they will support, followed by terrorism and foreign policy at 15%, the deficit at 11% and immigration at 10%.
Trump is by far the leader on the economy, judged best able to handle it by 49% of Republicans -- far ahead of Cruz's second-place showing at 11%.
And he's also seen as best able to handle terrorism, with 30% picking Trump, 20% preferring Cruz on the issue, 10% for Rubio and 7% for Bush.
The poll comes in the wake of the Paris terrorist attacks. About 88% of Iowa Republicans say they are "very worried" or "somewhat worried" about the possibility of a similar attack in the United States.
And those Republicans broadly oppose allowing Syrian refugees into the United States, with 81% wanting those refugees kept out and 15% saying they should be let in. The voters surveyed support sending U.S. ground troops to fight ISIS in Iraq and Syria, 73% to 22% opposed. By a margin of 83% to 9%, they say the United States and its allies are now losing to ISIS.
The poll surveyed 600 likely Iowa Republican caucus voters between November 16-22 and has a margin of error of plus or minus 4 percentage points.