A poll of registered Iowa Republicans released Tuesday shows that Mitt Romney, the party's 2012 presidential candidate now mulling another run in 2016, is currently leading a large field of prospective challengers. Wisconsin Gov. Scott Walker came in third, behind former Florida Gov. Jeb Bush.

The nation-leading Iowa caucus is still about a year off, but prospective candidates have already started to openly discuss the race. The Washington Post reported on Monday that Romney, a former governor of Massachusetts, is "almost certain" to run again, and has been discussing it with a number of close allies including former Minnesota Gov. Tim Pawlenty.

The poll by Gravis Marketing, a Florida-based pollster, found Romney leading a field of eight other potential candidates with the support of 21 percent of respondents. "Romney's name recognition and the loyalty Republicans have for their last nominee give him an opportunity that no one else has," said Doug Kaplan, the manager partner of Gravis Marketing.

Bush, the brother and son of the two former President Bushes, was next with 14 percent. Walker, recently re-elected to a second term as Wisconsin's governor despite a divisive first term that saw a failed recount attempt, was in third place with 10 percent. A number of additional potential candidates scored support below 10 percent. In order, they were: Mike Huckabee, Rand Paul, Ted Cruz, Paul Ryan, Chris Christie and Marco Rubio. Another 18 percent of poll respondents were undecided.

Ryan, a Wisconsin congressman, said Monday he would not run in 2016. The poll of 404 registered Iowa Republicans was taken Jan. 5-7.

The Iowa caucuses are tentatively scheduled for Jan. 18, 2016, though that date is not final. In 2012, Romney finished a close second in Iowa to former Pennsylvania senator Rick Santorum.

On the Democratic side, former Secretary of State and First Lady Hillary Clinton is seen as the prohibitive frontrunner. She has not made her candidacy official but is widely expected to announce this spring that she's running.

Clinton lost the 2008 Iowa caucus to then-Sen. Barack Obama, delivering an ultimately fatal blow to her candidacy.