SIOUX CENTER, Iowa — The campaign bus rolled to a stop in an Iowa parking lot. Earlier that day, recorded phone messages had been left with supporters in Sioux County, alerting them that, while the outdoor rally would proceed, unforeseen events would prevent the candidate from appearing. About 70 people — a large crowd by Iowa standards for so early in a presidential campaign — had shown up to stand in the sweltering heat anyway. It was the type of fervent anticipation long yearned for by backers of former Minnesota governor Tim Pawlenty, who has patiently courted Republican leaders and voters here for more than a year.