"I am going to keep my commitment," he said. "The American people have every reason to expect him to keep his commitment."
Obama spokesman Bill Burton on Thursday called public financing "an option that we wanted on the table," but said "there is no pledge" to take the money and the spending limitations that come with it.
Obama told reporters on Friday that it would be "presumptuous of me to say now that I'm locking myself into something when I don't even know if the other side is going to agree to it."
McCain said that if Obama becomes the nominee and decides against taking public money, he might do the same.
"If Senator Obama goes back on his commitment to the American people, then obviously we'd have to rethink our position," McCain said. "Our whole agreement was that we would take public financing if he made that commitment as well. And he signed a piece of paper, I'm told, that made that commitment."
Early in the race, Obama asked the Federal Election Commission whether he could raise general election money during 2007 but return it if he chose to accept the public funds.
Also, in response to a questionnaire in November from the Midwest Democracy Network, a group of nonpartisan government oversight groups, Obama said: "Senator John McCain has already pledged to accept this fundraising pledge. If I am the Democratic nominee, I will aggressively pursue an agreement with the Republican nominee to preserve a publicly financed general election."
McCain earlier this week turned down government matching funds for the primary to free him to spend more money as he prepares for a general election contest.
Last summer, McCain had asked to participate in the public system when his campaign, his fundraising and his poll numbers hit a low point that threatened to unravel his candidacy.
Though the FEC declared him eligible to receive $5.8 million in December, the money would not have become available until next month. By accepting the money, moreover, McCain would have been required to limit his spending for the primary to about $54 million - an amount the campaign was close to reaching now.
By not taking the money, McCain is free to raise more and to promote his presidential candidacy until the Republican nominating convention in September.
McCain would be the obvious beneficiary if he and Obama take the federal money for the general election because they would have to return money already collected. Obama has raised $6.1 million for the general, nearly three times as much as McCain's $2.2 million.
If the candidates reject public funds it would be historic rejection of the public financing system. No major party candidate rejected public funds for the general election since the system was put in place in the 1970s after the Watergate scandal.
Candidates who accept public funding are eligible for about $85 million, which is paid for by a $3 checkoff on IRS tax return forms.
McCain, who has all but sealed the GOP nomination, has focused much of his criticism on his Democratic rivals - Obama and Sen. Hillary Rodham Clinton. As Obama has gained the momentum, he has drawn a greater proportion of McCain's wrath.
Earlier this week, McCain suggested Obama has been lacking in providing policy details. "I've not observed every speech he's given, obviously, but they are singularly lacking in specifics," McCain said.
Campaigning in LaCrosse, Wis., McCain said he would propose a balanced budget in his first term if he is elected president - but not necessarily in his first year.
"I've got to give you some straight talk: I doubt, given the deficits we're running, that I can propose a balanced budget in the first year," McCain told a town hall meeting. "But that's my goal. It has to be our goal, because we're mortgaging these young people's future."
McCain said he would propose a balanced budget by the end of his first term if elected.
