Mr. Romney's campaign said he would still compete in Iowa's caucuses, now slated February, 2012. But the former Massachusetts governor's absence from the Aug. 13 straw poll will likely diminish his chances, some Iowa officials say, while raising the odds that whoever wins the poll will be best positioned to claim the status as Mr. Romney's top rival for the nomination. According to 2008 election-day surveys, 60% of Iowa caucus-goers called themsevles evangelical or born-again Christians. In the New Hampshire primary, the figure was 23%. In the general election that year, about a quarter of voters in both parties were...