Wisconsin Gov. Scott Walker continues to lead in Iowa among likely Republican presidential contenders, according to a new Quinnipiac University poll released Wednesday.

Twenty-one percent of likely Iowa caucus participants said they would vote for still-unannounced Walker if the caucus was held today, down a bit from the 25 percent he received in a February poll, but still ahead of the rest of the Republican field.

Sens. Rand Paul and Marco Rubio, who announced their presidential campaigns last month, tied for second place with 13 percent. Sen. Ted Cruz, who announced his campaign in March, received 12 percent, and 11 percent said they would vote for former Arkansas Gov. Mike Huckabee, who announced his campaign Tuesday.

Retired neurosurgeon Ben Carson, who also announced his bid this week, was at 7 percent, followed by former Florida Gov. Jeb Bush at 5 percent. No other candidate was near above 3 percent.

"The first few months of the Iowa Republican caucus race show Wisconsin Gov. Scott Walker as the early leader. U.S. Sen. Marco Rubio of Florida, on the strength of an impressive candidacy roll out, has moved from the bottom of the pack into a tie for second," said Peter Brown, assistant director of the Quinnipiac poll. "Former Florida Gov. Jeb Bush, who has yet to formalize his candidacy while he amasses what most expect to be a massive fund-raising lead, runs seventh with just 5 percent of the vote."

When respondents were asked which candidate they would definitely not support, Bush came in first at 25 percent. New Jersey Gov. Chris Christie was right behind with 20 percent and Paul followed with 10 percent in that category.

As for favorability ratings, Rubio scored the highest, with 69 percent of likely caucus-goers saying they viewed him favorably, while just 9 percent had an unfavorable view.

Walker got a 59 percent/11 percent favorable/unfavorable rating, and Paul received 59 percent favorable/23 percent unfavorable.

The February Quinnipiac poll saw Walker at 25 percent, Paul at 13 percent, Carson and Huckabee at 11 percent each, Bush at 10 percent, Cruz at 5 percent, and Rubio, former Sen. Rick Santorum and Christie tied at 4 percent.

The poll was conducted April 25 to May 4 among 667 likely caucus participants and has a margin of error of plus or minus 3.8 percentage points.