?Do we have any Aggies in the crowd?? asked Mr. Perry, summoning a voice from his days back on the yell squad at Texas A&M as he looked over a swarm of people who gathered to see his debut here as a Republican presidential candidate. ?We?ve got to have some Aggies in the crowd!?

With that, a cheer rose up and the audience burst into applause. The heckler, similar to one who irritated Mitt Romney and Michele Bachmann at the same spot a few days earlier, was swiftly silenced. As a satisfying smile stretched across his face, he quickly resumed telling voters why he believes he should be the next president.

?I get a little bit passionate,? Mr. Perry said. ?I think you want a president who is passionate about America ? that?s in love with America.?

The introduction of Mr. Perry as an aspiring presidential candidate unfolded in bite-size pieces, with fresh details emerging as he sauntered across the fairgrounds on the third day of his announcement tour. The path had already been well worn by his Republican rivals who camped out in the state last week, but he breezed in like a long-lost visitor, so confident that he blew kisses into a camera when asked about Mr. Romney. ?Give him my love,? Mr. Perry said.

The addition of Mr. Perry to the presidential campaign has changed the landscape of the Republican field ? particularly for Mr. Romney and Mrs. Bachmann ? while injecting a shot of vigor into the contest. Whether making up for lost time or feeling an itch to engage while he had a ready audience, Mr. Perry held a rolling conversation with reporters, interrupted again and again by people rushing over to thank him for joining the race. It continued at an evening stop in Cedar Rapids, when asked if Mr. Obama loved America, he said, ?You?d have to ask him.? He also said Federal Reserve Chairman Ben Bernanke would get ?ugly? treatment in Texas for his policies.

Asked about comparisons to George W. Bush, he said: ?I?m Rick Perry. He?s George Bush. Our records are quite different.? He added, ?I went to Texas A&M; he went to Yale. George Bush is not my opponent.?

Asked about Sarah Palin, he said: ?Sarah is a dear friend. She?ll make the right decision.?

But it was the questions about Mr. Romney that seemed to most engage Mr. Perry.

?Take a look at his record when he was governor. Take a look at my record,? Mr. Perry said. A few minutes later, he added: ?I wasn?t on Wall Street. I wasn?t working in Bain Capital,? forging directly at the critique that Mr. Romney has made against him ? that his credentials are limited to government service, not deep experience in the private sector.

(At nearly the same time in New Hampshire on Monday, Mr. Romney said it was critical to have experience from the ?real economy,? but he refrained from direct criticism of Mr. Perry. He added, ?I?ve learned how the economy works, and I believe that skill is what the nation is looking for.?)

The long-distance exchange, experience in the public sector versus the private sector, highlights a central argument that will be debated until Republicans choose a nominee next year to challenge President Obama. The bumper sticker message of Mr. Perry?s candidacy, ?Getting America Working Again,? is painted on his campaign bus, which on Wednesday will come within a few miles of Mr. Obama?s own bus tour.

?This president has been an abject failure when it comes to the economy,? Mr. Perry said.

For his part, Mr. Perry seemed pleased to take as many questions on the subject that came his way. He paused for a moment, as he reached a patch of shade on a warm afternoon, and told how he would lower the nation?s unemployment rate.