WATERLOO, Iowa — The leading Republican presidential candidates scrambled to take command of a new landscape on Sunday after Tim Pawlenty abruptly ended his campaign and a three-way race began taking shape to find a nominee who can emerge as the strongest challenger to President Obama.

While Gov. Rick Perry of Texas had hoped to turn the contest into a two-man duel with Mitt Romney, he starts by facing Representative Michele Bachmann, whose weekend victory in the Iowa straw poll reordered the top tier of candidates. On the second day of his announcement tour, Mr. Perry sent a subtle message: making his first Iowa appearance in her hometown, but not taking her on directly.

While Mrs. Bachmann, Mr. Perry and Mr. Romney each have emphasized cutting attacks on Mr. Obama, they now face the need to begin drawing distinctions with one another and set up what could be a long and hard-edged campaign for the party’s nomination.

Their pitches often overlap, with Mr. Perry and Mr. Romney presenting themselves as job creators, while Mr. Perry and Mrs. Bachmann appeal to social conservatives.