"The real gamble in this election is having the same old folks do the same old things and somehow expect a different result," Obama told a crowd packed into the Catfish Bend Casino in Burlington. "That's the definition of insanity: You keep on doing the same things over and over again and expect different results."
"People say to me they want me to sit at a table and negotiate with these people?" Edwards said to an enthusiastic crowd of about 1,000 in a Des Moines high school cafeteria. "Never. It will never happen. We're going to stand up to these people."
Things were even nastier on the Republican side, where Romney has taken to the airwaves attacking Huckabee.
Huckabee returned fire during a lunchtime appearance and news conference in Indianola. "It's one thing to attack us on our records," he said. "It's another thing to make it up."
Asked by a reporter whether he would vote for Romney if he won the Republican nomination, Huckabee said he could not support a Democrat over a Republican but pointedly declined to answer directly.
For his part, Romney largely ignored his GOP rivals on the stump, offering his promise of change. Speaking to about 75 people at a community center in Ottumwa, Romney said: "If there's ever been a time we need a change in Washington, it's now."
He mentioned his work as a business consultant and leader of the Winter Olympics in Salt Lake City, and his term as governor, saying he rescued Massachusetts from fiscal troubles by holding the line on spending. "I think I could do the same thing to Washington: Bring change," he said.
The message, however, is a tricky one for Republicans, said Emory University political scientist Alan Abramowitz.
"Democrats can talk about the kind of change that Democratic voters want," Abramowitz said. "But Republicans have a dilemma: How do you appeal to those core conservatives who are generally not as dissatisfied with things while, at the same time, appealing to the broader desire for change?"
Another twist is the way the two parties seem to have traded roles this election.
The Democrats have waged a fairly straightforward campaign, with a large field of challengers, led by Obama and Edwards, vying to topple the early front-runner, Clinton.
Republicans, by contrast, have shed their hierarchical tradition and watched as a series of candidates have struggled to break free of the pack, with different contenders holding the lead at different times in Iowa, New Hampshire and nationally.
"Democrats are a lot better at chaos than we are," said Republican Dan Schnur, who served as communications director for McCain's 2000 campaign. "We just don't have experience with this. As a result, you see a lot of voters and a lot of activists rushing from one candidate to the other trying to create a more familiar situation with one front-runner and several challengers."
In part, that reflects a broader struggle within the GOP to overcome Bush's low approval ratings, the loss of Congress last year and a split between economic and social conservatives.
The absence of a unity candidate has dampened Republican enthusiasm, as reflected in the crowds Saturday in Iowa. Whereas Clinton, Obama and Edwards appeared before hundreds of people at each stop, the GOP candidates -- including former New York City Rudolph W. Giuliani -- often drew audiences of 100 or fewer.
Terri O'Hara, a middle-school teacher, 57, was one of those who came out Saturday to see Romney in Ottumwa. She likes him, she said, because he seems capable and competent. But, she said, wincing, "I'm not sure if he's really down-to-earth."
She saw Huckabee on Friday and likes the personal connection she felt. But, O'Hara went on, "to be president of the United States is a major league ballgame, and I'm not sure if Huckabee is a major league player."
How will she decide by Thursday? "I've just got to do a little more thinking," she said. "And talking to people."
Times staff writers Maria L. La Ganga, Scott Martelle, Joe Mathews, Seema Mehta, Dan Morain, Peter Wallsten and Aaron Zitner and researcher Nona Yates contributed to this report.