Donald Trump often brags that he leads his Republican rivals in every poll, but Neurosurgeon Ben Carson has now surged up in the pack to tie Trump in Iowa, Monmouth University found in a poll out

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.

Trump and Carson are tied at 23%, according to the survey, making it the first time since July 26 that a poll in the first four states to select a Republican nominee did not find Trump substantially ahead of all other candidates.

Farther behind Trump and Carson, former Hewlett-Packard CEO Carly Fiorina, Senator Ted Cruz of Texas and Gov. Scott Walker of Wisconsin made up the next tier of candidates with Carly Fiorina, at 10%; Texas Sen. Ted Cruz, at 9%; Wisconsin Gov. Scott Walker, at 7%; former Florida Gov. Jeb Bush, at 5%; Ohio Gov. John Kasich and Florida Sen. Marco Rubio, each at 4%; Kentucky Sen. Rand Paul, at 3%; and former Arkansas Gov. Mike Huckabee and former Pennsylvania Sen. Rick Santorum, at 2%. No other candidates topped 1%.

The results are a stark contrast from Monmouth's last Iowa poll, taken before the first Republican debate. In late July, Walker led with 22%, Trump had 13%, Carson had 8% and Fiorina was at 3%.

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.

Trump had a much stronger lead earlier this month with several polls having him at 22%, Carson at 14%, Walker at 9%, Cruz at 8% and Fiorina at 7%.

The Monmouth poll follows a survey from The Des Moines Register over the weekend that showed Mr. Trump leading in the state, but revealed that Mr. Carson is gaining ground.

In Monmouth's survey, when voters said they had strongly made up their mind on who they were voting for, Trump led Carson 30% to 22%. But those with only slight preferences backed Carson 25% to Trump's 16%.

Monmouth surveyed 405 registered Republican voters and likely Iowa caucus-goers from Aug. 27 to Aug. 30 by live phone interview, with a margin of error of plus or minus 4.9 percentage points.

Monday’s poll also showed that Mr. Carson is leading Mr. Trump with people who consider themselves to be “very conservative,” with evangelical Christians, and with women.

Things do not seem to be going well for Former Gov. Rick Perry’s campaign who continues to slide further down or perhaps winding down his campaign.

Rick Perry struggling to raise funds to support his campaign has had his staff from paid to volunteer positions and now slashed to just a single staff member.

Mr. Perry led in the polls for a stretch during his first presidential bid four years ago. He spent significant time boning up on foreign policy and getting himself in physical shape for the grinds of the campaign trail.

However, he has failed to generate excitement among voters in a crowded and more competitive Republican field. A Monmouth University poll on Monday showed Mr. Perry with support of 1 percent of likely Republican caucusgoers in Iowa.

Most of Mr. Perry’s staff members have already joined other Republican organizations, with one signing on to work with former Senator Rick Santorum of Pennsylvania.

Mr. Perry had slashed his staff in South Carolina and other crucial primary states. Last his campaign chairman in Iowa, Sam Clovis left the campaign to join Donald Trump’s staff.