Despite Tim Pawlenty’s best efforts to lower expectations ahead of Saturday’s straw poll in Ames, Politico reports on why nothing less than winning will be good enough:

Iowa caucus veterans say the calculus for Pawlenty is both simpler and harsher: Anything short of a flat-out win will make it exceedingly difficult for him to claim momentum in a rapidly accelerating 2012 primary race.

With all the resources Pawlenty has committed to Iowa, he’ll need a real trophy to carry back to his core supporters, as well as the undecided donors he desperately needs to win over. Pawlenty has struggled to raise money throughout his 2012 bid, and could end up running on fumes if he can’t demonstrate his viability in Iowa this weekend.

Rick Perry will be making an announcement on Saturday designed to compete with the result of the straw poll, and his entry into the race seems assured. That makes things much harder for Pawlenty for several reasons, including Perry’s instant ability to take over the anti-Romney role that Pawlenty has tried and so far failed to fill. It also means that no one is going to be paying much attention to any of the candidates finishing third or worse. To make any significant impression on Saturday, Pawlenty needs to win. At least, that’s what some veteran observers in Iowa believe:

David Yepsen, a former longtime Des Moines Register columnist, said that Pawlenty has “got to have something to crow about” after Ames, especially in light of Rick Perry’s imminent entry into the campaign.

“What’s gonna bust through the clutter of Rick Perry on Saturday? Winning,” said Yepsen, suggesting that a bad week for Pawlenty “could knock him out of the race.”

Supposing that Pawlenty manages to surprise everyone and win the straw poll, it will be what he needs to survive, but it may not be enough to propel him into the top tier now that Perry is coming into the race. It’s clear that Pawlenty’s woes over the last several months have been an important factor in leaving an opening for Perry to enter the race. If Pawlenty had appeared to be more successful over the last few months, the demand for another candidate would not be nearly as great, and the opportunity for Perry to enter the race with immediate support as the alternative to Romney would not have presented itself. Once Perry is in the race, Pawlenty’s “underdog” routine won’t help him very much, since this just confirms that Romney and Bachmann skeptics should turn to Perry instead. He won’t be able to rely on his executive experience argument as heavily, because Perry can boast of having served even longer as governor. Just as Bachmann was able to outflank Pawlenty as a fiscal and social conservative, which made her a better alternative to Romney up until now, Perry matches and perhaps exceeds Pawlenty in all of the areas that are supposed to make Pawlenty preferable to Bachmann. Saturday could be the day when Pawlenty’s campaign revives only to be reminded that the revival has come too late.