Making his case againstwithout naming her, Obama said, "The real gamble in this election is playing the same Washington game with the same Washington players and expecting a different result."
Clinton has emphasized her Washington experience - as first lady and then as a senator from New York - though she, too, calls herself the candidate of change.
"In this election, it is time to turn the page," Obama said. "In seven days, it is time to stand for change."
Obama, a senator from neighboring Illinois, is spending the final week of the Iowa campaign speaking to voters in small towns across the state. But one week before the Jan. 3 caucuses, he came to the capital of Des Moines, where representatives of the world's media were gathered, to deliver his "closing argument" speech.
It was overshadowed by news from Pakistan that former Prime Minister Benazir Bhutto had been assassinated.
Her death had the potential to bring terrorism back to the center of the presidential campaign, and Obama began his speech by mourning her loss and vowing that the United States "will be steadfast in our desire to end the types of terrorist attacks that have blighted not just Pakistan but the rest of the world."
Obama's national co-chairman, retired Gen. Merrill "Tony" McPeak, the former chief of staff of the Air Force, also raised the killing in his introduction.
"The events of this morning bring back with great clarity how important these national security issues are," he said. McPeak supported President Bush in the 2000 election, but he now says Bush lacks the intelligence that Obama would return to the Oval Office.
"We've been running a giant experiment in this country in the last seven years, testing the idea that it doesn't make that much difference if the guy in the White House is not that bright. And the results are in," McPeak said. "No matter what happens next January, there is going to be a big jump in the IQ in the Oval Office."
Obama said he decided to run for president less than halfway through his first term in the U.S. Senate because the nation is at a "defining moment" and Americans are hungry for a new kind of politics that speaks to their hopes.
"You know that we can't afford four more years of the same divisive food fight in Washington that's about scoring political points instead of solving problems, that's about tearing your opponents down instead of lifting this country up," Obama said, his voice rising over the cheers of his supporters. "We can't afford the same politics of fear that tells Democrats that the only way to look tough on national security is to talk, act, and vote like a George Bush Republican, that invokes 9/11 as a way to scare up votes instead of a challenge that should unite all Americans to defeat our real enemies."
Obama has been criticizing Clinton for supporting the president in his move to war with Iraq four years ago and his tough line against Iran today - although she now opposes the war and says she wants Bush to pursue diplomacy with Iran. Obama's argument for change is up against Clinton's call for voters to avoid electing another president like Bush who isn't experienced in foreign policy.
While such "calls for change" are politically popular, they sometimes run counter to reality, CBSNews.com senior political editor Vaughn Ververs said.
"It's unclear just how much 'our politics' can change in the present environment," Ververs said. "Real divisions exist on real issues -- foreign policy, the makeup of the judiciary, taxes, national security, immigration and social values to name just a few. Not all of them break down strictly along party lines but most are fodder for the talk shows, blogs and shouting matches that have come to dominate American political discussion. And none will be decided without a fight." (Read more from Ververs in Horserace.)
The Clinton campaign responded to Obama by saying Iowans will choose which candidate can provide new leadership in a time of war and troubled economy.
"There are big stakes in this election," said Clinton spokesman Phil Singer. "Iowans are going to pick the candidate best able to make the change we need starting on day one and that candidate is Hillary Clinton."
Obama rejected the sentiment that Clinton could bring a new beginning.
"You can't at once argue that you're the master of a broken system in Washington and then offer yourself as the person to change it," he said. "You can't fall in line behind the conventional thinking on issues as profound as war and then offer yourself as the leader who is best prepared to chart a new and better course for America."
Clinton and Obama are in a three-way race for the Iowa caucuses with 2004 vice presidential nominee. Edwards has been arguing that he's the stronger voice to confront entrenched interests in Washington
"Now, there are others in this race who say that this kind of change sounds good, but that I'm not angry or confrontational enough to get it done," Obama said. "Well, let me tell you something, Iowa. I don't need any lectures on how to bring about change, because I haven't just talked about it on the campaign trail. I've fought for change all my life."
Edwards planned to give his own speech calling for a different kind of change.
"Compromise and conciliation is the academic theory of change," Edwards said in an excerpt distributed in advance by his campaign. "It just doesn't work in the real world. Fighting for conviction is the historic reality of change."