The Ames straw poll is sometimes referred to as the "graveyard" of campaigns, because although the winner doesn't always go forward to victory, candidates who perform very poorly often quit after Ames. Apparently Pawlenty had decided in advance that he would only remain in the race if he placed first or second (or perhaps a very close third) at Ames.
The fact that Pawlenty is the first candidate to quit is ironic, because his optimal role in the primary was to be the "last man standing," the candidate who remained after the rest of the field destroyed itself. But by the end of June, Elephant Watcher noted that Pawlenty's campaign was schizophrenic and showed signs that it didn't have a proper strategy. Rather than avoiding enemies and playing the role of the consensus candidate, Pawlenty was increasingly attacking others.
Pawlenty's decision to quit early was also ill-advised. A candidate with the "last man standing" strategy needs patience, as he is unlikely to pick up steam until the end of the campaign. Instead, Pawlenty quit the race before it had even begun. During Phase Two of the primary, even voters in early states like Iowa and New Hampshire are not yet paying close attention to the race.
This serves as a good demonstration of the fact that political candidates and their advisors are very fallible. One might expect those who run for the presidency to know quite a lot about campaign strategy. But as Rudy Giuliani showed in 2008 with his "Florida first" strategy, it's possible for candidates to be completely clueless. Pawlenty's inability to grasp what kind of candidate he was doomed his campaign.
A detailed analysis of how Pawlenty's departure affects the race will be the subject of a future post.