Senator Barack Obama has moved in recent weeks to sharpen his tone noticeably as he fights for the Democratic presidential nomination, increasingly drawing sharp contrasts with his rivals and seeking to turn criticism of his foreign policy credentials into a fresh argument for change.
The recalibration of the campaign is a marked departure from a laid-back tone Mr. Obama often had taken in the first six months of his candidacy. It comes as he is working to persuade voters of his judgment and erase perceptions among party leaders in states like this that Senator Hillary Rodham Clinton of New York is establishing herself as the front-runner after a series of debates and what some Democrats have viewed as slip-ups by Mr. Obama.
Laying the groundwork for a debate in Iowa on Sunday, and at the start of a three-day bus tour across the state on Thursday, he seized at an opportunity to highlight differences with Mrs. Clinton, a level of engagement he waved off in the past. But the window is narrowing for the most enthusiastic party activists to choose sides, and Mr. Obama is seeking to make a stronger case for himself.
“There is — not just with Senator Clinton, but a lot of my opponents — a premium on reciting the conventional wisdom in Washington,” he said at a news conference before speaking here in southwestern Iowa. “And that’s what passes for experience.”
Barely a week has passed since February when Mr. Obama has not made one — or more — visits to Iowa to introduce himself to voters who will open the presidential nominating contest in January. In hundreds of appearances, perhaps no question has been asked more than whether he has the experience to be president.
Mr. Obama raised the issue on his own here Thursday as he sought to push back any concerns, arguing that judgment was more critical than experience.
A dispute between Mr. Obama and Mrs. Clinton over foreign policy, which began last month in a debate in South Carolina, has provided a new campaign narrative for Mr. Obama. He has tried to turn Mrs. Clinton’s critiques on him back against her by reminding voters of his opposition to authorizing the Iraq war in 2002, when he was a state senator in Illinois and suggesting that he represents a break from the business-as-usual ways of Washington represented by both President Bush and Mrs. Clinton.
“It comes down to a question that the American people are going to have to ask,” Mr. Obama said after a recent campaign stop in Le Mars. “Are you looking simply to replace George Bush and otherwise tweak the system and continue on a process of very modest incremental change? Or do you think that the problem is more than just George Bush and it’s a problem with how Washington works?”
The message, aides say, is crafted either to lure voters to Mr. Obama’s side or to keep them undecided a bit longer. By contrast, Mrs. Clinton seldom — if ever — makes reference to her rival as she campaigns.
“Hillary Clinton has fought for change her whole life, and she is the candidate with the strength and experience to make change happen starting in 2009,” said Phil Singer, a campaign spokesman, when asked Thursday to respond to Mr. Obama.
It remains an open question whether the sharper tone — a departure from his more professorial air early in his candidacy — carries any risks for a candidate who pledged to campaign on a message of hope and a new kind of politics. Mr. Obama has told associates he finds the burst of aggressiveness to be liberating.
Even Michelle Obama presented a contrast here on Thursday as she introduced her husband in an open-air barn at the Cass County fairgrounds. She told a crowd of more than 200 people that family values and trust were important in the next presidential candidate.
“Our view is that if you can’t run your own house, you certainly can’t run the White House,” Mrs. Obama said.
Later, she added: “This election is about truth and authenticity. There is nothing more important than your word. Truth does matter.”
When he took the microphone, Mr. Obama used similar phrasing, saying, “Part of the change, by the way, is telling the truth to the American people about the very serious and difficult challenges and choices that we face.”
Despite subtle changes in tone during this summertime stretch of the long campaign for the White House, Mr. Obama is still seeking to demonstrate that he is, as he likes to say, a candidate of hope. Even the root beer floats served to supporters sat beneath a homemade banner, “Floats 4 Change.”
In a question-and-answer session here, a woman rose from her seat and declared, “You excite the hell out of me, and I haven’t been hopeful for a long time!”
As presidential hopefuls in both parties descended upon Iowa this week, several voters said they attended multiple campaign events to size up the contenders. While many Iowans say they will wait until the final weeks to select a candidate, local officials and party activists are being vigorously wooed to be onboard shortly after Labor Day.
Patrice Watson, a 55-year-old medical researcher from Council Bluffs, said she saw Mrs. Clinton on Tuesday and Mr. Obama on Thursday. She had yet to make up her mind, Ms. Watson said, but was eager to hear the distinctions.
“She’s a very good speaker,” Ms. Watson said of Mrs. Clinton in an interview. “She seems very knowledgeable. She really knows how things work, and she’s very smart and experienced.”
She added: “But I’m not sure that she’s really committed to the sort of radical change that I think we need. We need things to really change — the way Washington works, the way decisions are made in this country.”
As she walked away, Ms. Watson turned back and said, “Right now, I’m likely to be hanging an Obama sign in the window, but I always sleep on things before I decide for sure.”