AMES, Iowa — The race for the Republican presidential nomination came into sharper focus on Saturday as Gov. Rick Perry of Texas declared his candidacy in South Carolina and Michele Bachmann won a closely watched poll of voters in Iowa.

With Republican enthusiasm swelling over the prospect of defeating President Obama next year, thousands of party activists converged here for the Iowa straw poll. The outcome, combined with the entrance of Mr. Perry, could help reorder the top tier of contenders, with Mrs. Bachmann and Mr. Perry positioned to challenge the perceived front runner, Mitt Romney, and each other.

Ron Paul of Texas, whose libertarian views put him at odds with many Republicans, finished slightly behind Mrs. Bachmann in the straw poll. Tim Pawlenty, a former governor of Minnesota who argued that Mrs. Bachmann was not sufficiently qualified to be the party’s nominee, finished a distant third.

The results of the straw poll provide only a snapshot of the race, but along with Mr. Perry’s announcement, represented a significant reshuffling of the campaign and highlighted a deep uncertainty among Republicans over who would be the strongest nominee. Republicans sense a new opportunity to win back the White House, but there was little clarity about whether voters would choose someone from the party establishment, an outsider or a hybrid.