AMES, Iowa (AP) — After months of laying the groundwork for their campaigns, the Republicans who want to unseat President Barack Obama in 2012 are finally getting a chance to see how they rate with party activists.

A straw poll Saturday in Iowa, the leadoff caucus state, marks the first time Republican voters weigh in on the GOP presidential field. Results from the poll, an early measure of viability, were due out just hours after Texas Gov. Rick Perry's expected announcement in South Carolina that he was joining the race.

The poll results are nonbinding, amount to a popularity contest and offer candidates a chance to test their get-out-the-vote organizations with the thousands of activists who pay to attend the event. Nonetheless, the outcome probably will provide a road map for the Iowa campaign heading into the caucuses that are just four months away.

To the state party chairman, Matt Strawn, the poll "is the first measurable proving ground" for the field.

Former Massachusetts Gov. Mitt Romney leads national polls and many states' surveys for the chance to challenge Obama next fall. But there is no shortage of rivals looking to emerge as the top alternative to Romney, who lost the nomination in 2008 to Arizona Sen. John McCain.

The candidates with the most to lose in the poll were two Minnesotans competing to fill that role: Former Gov. Tim Pawlenty and U.S. Rep. Michele Bachmann.

They clashed during Thursday's debate in Ames, when Bachmann said Pawlenty "sounds a lot like Barack Obama if you ask me" and Pawlenty accused her of having "a record of misstating and making false statements."

Pawlenty, languishing in early Iowa polls, hoped a victory would prove he's a strong player in the race. Bachmann wanted to build on momentum she's enjoyed since entering the race this summer.

"For some people, this is make or break," tea party activist Ryan Rhodes said.

Nine candidates were on the ballot. Voting was to run for six hours during the daylong political festival on the campus of Iowa State University.

Romney won the straw poll four years ago but isn't actively competing this time, and former Utah Gov. John Huntsman, who has bypassed Iowa almost entirely, weren't scheduled to be in the state. Both spent Friday in New Hampshire.

Perry and former Alaska Gov. Sarah Palin, who made a splash Friday when she visited the state fair, weren't on the ballot. But their supporters planned write-in campaigns that could outpace candidates who have spent months trying to line up supporters to participate.

Others on the ballot included former U.S. House Speaker Newt Gingrich of Georgia, U.S. Rep. Ron Paul of Texas, former U.S. Sen. Rick Santorum of Pennsylvania and businessman Herman Cain.

"Part of it is whether the message resonates, but the bottom line is you better be pretty well organized," Iowa Christian Alliance head Steve Scheffler said.

A circus-like atmosphere was in the making on the college campus. Campaigns erected giant tents for live music and prepared tangy barbecue to court activists.

Republicans wouldn't speculate how many people they expected would spend $30 each to attend the event. Turnout in past has ranged from 14,000 to 23,000.

The straw poll has a mixed record of predicting the outcome of the precinct caucuses.

In 2008, Romney won the straw poll, but the big news was the surprising second-place showing of former Arkansas Gov. Mike Huckabee. Huckabee won the Iowa caucuses, but dropped from the race soon after. McCain, who eventually won the nomination, didn't compete in the straw poll and finished in 10th place.

Poor showings usually force some candidates, mostly those who are not well-known and are struggling to raise money, to abandon their bids, and that could happen this year, too.

GOP strategist Rich Galen called it "a winnowing process and that might happen to Pawlenty if he doesn't do well. It sort of serves as a filter to clean out the candidates who probably shouldn't be there in the first place