Ben Carson ties Donald Trump in new Iowa poll

Washington (CNN)For the first time in more than a month, mogul Donald Trump is not leading the field in one of the first four nominating states in the Republican primary, according to a new poll.

Neurosurgeon Ben Carson has surged up in the pack to tie Trump in Iowa, Monmouth University found in a poll out Monday.

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 were former Hewlett-Packard CEO 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%.

A CNN/ORC poll taken earlier this month found a similar top tier of candidates, but with a much stronger lead for Trump. In that poll, Trump was at 22%, Carson was at 14%, Walker was at 9%, Cruz was at 8% and Fiorina was at 7%.

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.