TIFFIN, Iowa — At the outset of a crucial week in the presidential campaign, the Republican candidates are increasingly turning on one another as they try to overcome their own weaknesses and capitalize on the vulnerabilities of President Barack Obama.
A sense of urgency is rising for contenders to make favorable impressions at a debate Thursday, the first time Jon M. Huntsman Jr. will join his rivals on stage. That will be followed by the Iowa Straw Poll on Saturday, a ritual that provides an early test of organizational strength that could accelerate or dampen the aspirations of some candidates.
The full Republican field will descend upon Iowa this week to court voters and present their arguments to party activists, some of whom say they are eager to build the drumbeat of criticism about Obama into a candid discussion of which candidates could best defeat him.
"The opportunity is right there for us," Tim Pawlenty, a former governor of Minnesota, told a Republicans here on Friday. "The main way we're going to goof this up, as Republicans, is to nominate the wrong candidate."
As Pawlenty made his way across Iowa this weekend, he warned Republicans against supporting someone without executive experience. His audiences nodded in agreement. Yet the target of his criticism, Rep. Michele Bachmann of Minnesota, drew booming applause from far larger audiences when she pledged to confront the establishment, declaring, "Behind this dress is a titanium spine!"
And looming over the race is the question of whether Gov. Rick Perry of Texas will join the field this month, as he has signaled to donors, activists and party officials. His entry could deliver a jolt to the race, but it remains an open question how long he could maintain the same level of curiosity and intrigue he now holds.
"Everybody's waiting for some ideal candidate to get in," said Sen. Charles E. Grassley, R-Iowa, who fielded a call from Perry last month. He added, "I told him to come to Iowa now and start campaigning like you campaign for sheriff."
As the candidates scramble to test their messages and gain attention, a majority of their arrows have been aimed at Mitt Romney, a former governor of Massachusetts.
Huntsman, a former governor of Utah, assailed Romney's leadership on the debt ceiling debate. Rick Santorum, a former senator from Pennsylvania, questioned his convictions on conservative social issues. Even the former GOP vice presidential candidate Sarah Palin, who has yet to rule out a run, suggested that he placed his finger to the wind instead of leading by example.
Romney, who has maintained a disciplined silence, is preparing for rougher treatment in the debate Thursday than he received in his first one in June. His advisers say he is opening a new campaign phase by making more public appearances after several months of lying low. He is not overtly campaigning for the straw poll, but is waging a stealth campaign in Iowa to keep his options open until the field clarifies.
Republicans sense a fresh opening surrounding Obama, after painful budget talks, sustained high unemployment and lowering the nation's credit rating. ("It happened on your watch, Mr. President," Bachmann said Saturday.) But that is offset for some by a lack of consensus over who would be the strongest — or least flawed — Republican nominee.
Here in Tiffin, a handful of Republican candidates delivered speeches and mingled with 300 party activists at a picnic supper Friday. As Pawlenty, Santorum and Newt Gingrich, a former House speaker, worked the room, several people stood out in the crowd. They wore burnt orange shirts printed with white lettering: "Americans for Rick Perry."
It was yet another sign that Perry was inching closer toward entering the race and would not be starting a campaign from scratch. The group, formed by friends who say they are operating independently, is urging people to attend the straw poll and support Perry as a write-in candidate, since his name will not be printed on the ballot.
The political merits of the straw poll are limited, particularly for predicting results of the caucuses next year. But the results — fair or not, considering only a few thousand people will take part in Ames — are likely to accelerate or extinguish the ambitions of some candidates.
Rep. Ron Paul of Texas, who has wide appeal among voters with a libertarian strain, is aggressively pursuing a strong finish at the straw poll. He has been working to expand on the deep support of his core followers and has encouraged them to send the party a message. Sen. Rand Paul of Kentucky, his son, will arrive this week to help.
The urgency in the voices of the candidates made clear how high the stakes are.
As Pawlenty concluded question-and-answer sessions with voters on Friday, he asked for their support — not in the caucuses, but more immediately, at the straw poll. He urged Republicans to take seriously his admonition to elect someone who could go the distance, not a candidate with more limited appeal.
"Before you fuel this rocket and launch it out of here," Pawlenty said, "make sure it can get all the way to the destination."
Pawlenty has moved most of his campaign organization to Iowa from his national headquarters in Minneapolis. He has invested nearly all of his limited finances on television commercials and campaign material to identify supporters and persuade them to go to Ames, where a strong finish in the straw poll could move him beyond his summertime slump.
The debate on Thursday evening, being televised by Fox News, also offers an opportunity for Pawlenty to avoid the missteps that he acknowledged making at the last debate when he backed away from challenging Romney on health care. That debate hurt his fundraising, his aides said, even as Bachmann's steady performance helped propel her candidacy.
Bachmann is also aggressively working to win the straw poll, which will provide a key test on whether she is able to turn the enthusiasm surrounding her candidacy into actual support. Her crowds are routinely larger than those of her rivals, but her campaign stops are staged as events more often seen in the general election, where a high priority is placed on appearances
For two days in a row, she was late arriving at campaign events in the warm afternoon sun and humidity. Her blue campaign bus was idling nearby, but as the crowd waited for her to arrive, an aide was busy arranging the crowd, saying into a loudspeaker, "We need to fill in the crowd and make it look nice and tight for our national media guys."
She also sought to lower expectations, which have soared since she entered the race two months ago.
"We're at a distinct disadvantage from an organizational perspective, but the response that we've had from Iowans has been overwhelming," Bachmann said. She also told the crowd that the White House was carefully watching her campaign, declaring, "They fear my candidacy more than any other."