Clinton advisers seized on the shift, suggesting it highlighted Obama's pattern of making promises to voters and revising them later as circumstances change.
"We don't need lectures on campaign finance from a campaign that's accepted more money from lobbyists than any other Republican or Democratic candidate who's run for president," Obama spokesman Bill Burton responded. "This is a question we will address if and when Obama is the nominee."
Clinton next heads to Ohio, where her husband, former president Bill Clinton, was already campaigning on Sunday.
After winning eight straight head-to-head contests, Obama led the chase for nomination delegates 1,280 to1,218. It takes 2,025 delegates to secure the presidential nomination at the party's convention this summer in Denver.
On the Republican side, McCain, in a taped interview aired on Sunday on ABC television's This Week, said he was confident he could persuade Americans that the Democratic candidates' call for an early Iraq withdrawal would result in a "catastrophe" with al-Qaeda claiming victory.
McCain denied the Democratic candidates' argument that voting for him would in essence amount to a third Bush term.
"How am I different? ... Climate change is an issue. Spending is another issue," McCain said, referring to his support for measures to combat global warming and wasteful government spending.
"I can out-campaign them, and I can out-debate them, and I can out-perform them ... My vision is more in keeping with the majority of Americans," said the 71-year-old Arizona senator, a former Vietnam prisoner of war
He added that he is "making progress" in solidifying his support among conservative Republicans. He said there will be "no new taxes" during his administration if he is elected president.
McCain was expected to receive an endorsement from former president George H.W. Bush in Houston, Texas, yesterday, Republican officials said. The endorsement is a further nudge for conservative activists to get over their distaste for McCain, and for former Arkansas governor Mike Huckabee to quit the race.
But Huckabee, an ordained Baptist minister, has refused to drop out until McCain secures the 1,191 delegates needed to secure the nomination at the party's convention this September in Minneapolis-St. Paul, Minnesota.
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