Despite leading the GOP pack in Iowa, Wisconsin Gov. Scott Walker has been losing steam.

Walker, who has plans to join the Republican presidential field on July 13, is polling at 18 percent from likely GOP caucus participants in Iowa, according to a new Quinnipiac University Poll.

Though that puts him ahead of a crowded Republican field, it is a noticeable decrease from the 21 percent he had two months ago. And in February, Walker had 25 percent.

"Those who thought the Republican race in the Iowa caucuses might begin to clarify itself better think again," said Peter Brown, assistant director of the Quinnipiac University Poll. "As even more candidates toss their hats into the ring, the race has gotten even more muddled."

The fight for second place remains a tossup.

Retired neurologist Ben Carson and real estate mogul Donald Trump, both first-time candidates, tie for second place with 10 percent of the vote. Just behind are Sens. Ted Cruz of Texas and Rand Paul of Kentucky, at 9 percent. Former Florida Gov. Jeb Bush garners eight percent, followed by Florida Sen. Marco Rubio with seven percent.

Former Arkansas Gov. Mike Huckabee, who won the 2008 Iowa caucuses, is at five percent, followed closely by former Texas Gov. Rick Perry and former Pennsylvania Sen. Rick Santorum (who won the 2012 caucuses) both at four percent. Former Hewlett-Packard CEO Carly Fiorina garners three percent, tied with Louisiana Gov. Bobby Jindal, who entered the race last week while the poll was being conducted.

Ohio Gov. John Kasich, who is expected to enter the race on July 21, garnered two percent of the vote, followed by New Jersey Gov. Chris Christie — who declared his candidacy on Tuesday, before the poll was conducted — with just one percent of the vote. South Carolina Sen. Lindsey Graham is also at one percent. Former New York Gov. George Pataki garnered no support.

The telephone poll of 666 likely Iowa Republican caucus participants was conducted June 20-29 with a margin of error of plus or minus 3.8 percentage points.