"It was something to behold," he said.
Relishing this first opportunity to fire back at the opposition party's gathering in Tampa, Mr. Obama said the agenda the Republicans offered was more often than not "better suited for last century."
"It was a rerun," Mr. Obama said of the policies offered up at the GOP convention. "We'd seen it before. You might as well have watched it on a black and white TV."
Addressing some 10,000 people on a farm in this town outside Des Moines, the president argued, "There was a lot of talk about bold truths and hard choices" at the GOP event. But, he said, "Nobody actually bothered to tell you what they were."
Mr. Obama said Romney "did not offer a a single new idea - just retreads of the same old policies that have been sticking it to the middle class for years."
Aides had said Mr. Obama didn't watch any of the Republican National Convention, but it was clear from his attack lines today that he had read and heard about what had gone on there.
He told his audience that his speech at the Democratic Convention in Charlotte next Thursday would offer "a better path forward."
He said it's "a path that grows this economy, creates more good jobs, strengthens the middle class."
His rally, at the Living History Farms, kicked off four days of campaigning, billed as the "Road to Charlotte." It will take him to events in Iowa, Colorado, Ohio and Virginia, swing states all, preceding his arrival in the convention host city on Wednesday.
Today's event marked his 13th visit to Iowa since taking office, his seventh this year and third in as many weeks.
With only six electoral votes to its name, Iowa has drawn more presidential attention this campaign than states with three times as many.
Mr. Obama won Iowa handily in 2008 by a margin of 54 percent to 44 percent, but recent polls show him in a tight race he's determined to win.
He told his crowd that Iowa was "where it first began" for him in the early days of the 2008 campaign, and that his supporters there had kept him going when the pundits were writing him off.
It was during his Iowa bus trip last month that Mr. Obama visited a couple of beer pubs and spoke of the home-brewed beer he was making at the White House using honey from bee-hives on the South Lawn.
His comments generated considerable interest and today, the recipes for two kinds of White House beer were posted on the White House website under the headline "Ale to the Chief."