The candidates have offered different assessments of how the straw poll results could affect their presidential hopes.
Most blunt was Thompson, who has visited each of Iowa's 99 counties and said he must do well to sustain his campaign. "If I don't come in first or second, that's not 'well,' " he told reporters here at the Iowa State Fair.
Huckabee and Brownback have avoided setting such benchmarks, but finishing deep in the pack would raise questions about whether they stay in the race.
Three other long shots -- Reps. Duncan Hunter of El Cajon, Tancredo of Colorado and Ron Paul of Texas -- have insisted they will persevere no matter the results.
Tancredo is counting on his tough posture on immigration to appeal to conservatives, and he pressed his case Friday on talk radio. He voiced amazement at "illegal Spanish-speaking people" in Iowa.
"When I talk to Iowans, they are frustrated; they are mad," said Tancredo, whose latest Iowa radio ad urged listeners to "join Tom's army against amnesty."
Others have focused on abortion as the key to winning the culturally conservative voters who dominate the GOP in Iowa. After a pep talk to volunteers at his Des Moines headquarters Thursday, Brownback called for the prevention of abortions when prenatal tests diagnose Down's syndrome.
"We're saying don't kill the child," he said. "Give the child to us, and we'll get the child adopted."
Brownback also took new shots at Romney for supporting abortion rights before he started running for president as an antiabortion candidate.
"He's changed positions on core issues, and I think we need people to lead on these issues, not move around," Brownback said.
For his part, Romney was focused Friday on lowering expectations for a landslide in the straw poll.
"Obviously, winning would be terrific, but who knows," Romney said while flipping pork chops on a grill at the state fair alongside his wife, Ann. The couple wore matching blue denim aprons with "The Other White Meat" printed on the front. Afterward, they admired pigs, cows and lambs in nearby pens.
Romney advisors described the straw poll as a "dress rehearsal" for the vast organizational task needed to win support at the Iowa precinct caucuses that will kick off the presidential nomination process.
Support in Ames costs money, and Romney's heavy spending on the straw poll has heightened prospects for embarrassment if his performance falls short.
