WASHINGTON — The Republican presidential field will likely shrink by the end of this weekend following an Iowa straw poll that carries the last hopes for a litany of long shot contenders.

The nonbinding Ames straw poll, a state Republican fund-raiser scheduled for Saturday, has traditionally been the first major test of campaign strength and organization in the White House race. This year, however, two leading candidates, Mayor Giuliani and Senator McCain of Arizona, are not actively competing, making the poll essentially a race between the overwhelming favorite, Mitt Romney, and everyone else.

The result is that although the former Massachusetts governor has poured millions of dollars into advertising and organizing for the poll, he may have more to lose than to gain. Anything less than a convincing victory could be seen as a significant blow to his campaign, particularly with surveys showing him with a double-digit lead in Iowa. In a University of Iowa poll released Wednesday, he had a 16-point lead over Mr. Giuliani, 27% to 11%.

For the hopefuls chasing him, Saturday’s straw poll is a make-or-break moment in the race. In 1999, George W. Bush’s victory in Ames prompted four of the eight Republican contenders to drop out of the race.

A strong second- or third-place finish could breath new life into one of the half-dozen campaigns that have gained little traction or notice thus far, giving them momentum heading into Iowa’s first-in-the-nation caucus in January.

At least one candidate, a former Wisconsin governor, Tommy Thompson, said that if he does not finish first or second, he will end his campaign. A Colorado congressman, Rep. Tom Tancredo, and a former Arkansas governor, Michael Huckabee, have made similar statements, although Mr. Huckabee yesterday backed away from a firm declaration.

“Rather than show that I’m no. 1, no. 2, no. 3, I’ve got to show that of nine candidates, I’m closer to the top than I am the bottom,” Mr. Huckabee said in a conference call with reporters.

Senator Brownback of Kansas is also making an aggressive push for the straw poll, although his campaign said he will stay in the race regardless of where he finishes.

“Senator Brownback hopes to do well, but he’s in the race for long haul and doesn’t consider the straw poll a make-or-break event,” a campaign spokesman, John Rankin, said yesterday.

The format of the poll, in which Iowa residents must pay $35 to participate, puts a premium on organization and funding, as campaigns bus in supporters from around the state by the hundreds.

A big question is how many votes will go to Messrs. Giuliani and McCain, as well as to a former Tennessee senator, Fred Thompson, though they are not attending or participating. One aide to a rival campaign predicted that Mr. Giuliani could finish as high as second after ramping up his presence and operations in the state recently, including a four-day swing this week. “A second- or third-place finish would be a great headline for them,” the aide said. “They’ve hedged their bets extremely well.”

Mr. Giuliani’s campaign announced in June that it would not participate in the straw poll, angering the state party and signaling that he was increasingly focused on the larger primary states, such as New York, Florida, and California, that have moved up their elections.

The former mayor has made a renewed effort in Iowa following the near-collapse of Mr. McCain’s campaign. Yet an aide to Mr. Giuliani said yesterday that the campaign would have no official presence at the straw poll and that any support he receives will be “organic.”

A potential dark horse on Saturday is Rep. Ron Paul of Texas. An anti-war Republican who ran on the libertarian ticket in 1988, Mr. Paul entered the race as a fringe candidate but has amassed a considerable Internet following and surprised the political world by ending the second quarter with more cash on hand than Mr. McCain, the early front-runner.

Mr. Paul’s campaign has bought just 500 tickets for supporters — compared to thousands that Messrs. Romney and Brownback have purchased, but the congressman is running radio ads in Iowa. His campaign has even urged supporters to take up the Romney’s campaign’s offer of a free ticket and lift only to cast their ballot for Mr. Paul.

“We want to demonstrate that we can bring bodies and that we’re not just an Internet phenomenon,” a spokesman, Jesse Benton, said.