John McCain threw down the gauntlet with Barack Obama for the second day in a row Friday, this time accusing the Illinois senator of trying to renege on a pledge to accept public financing in a general election campaign, a move that could limit Obama's vast and swelling financial resources.

Obama's campaign has said it will make no promise to use the public funds for the November election, even though the candidate previously indicated a desire to do so. The rules would restrict him to $85 million after the August convention. In contrast, Obama pulled in $32 million in January alone, while McCain pulled in $13 million.

"I made the commitment to the American people that if I were the nominee of my party, I would go the route of public financing. I expect Senator Obama to keep his word to the American people as well," McCain said in Oshkosh, Wis. "This is all about a commitment we made to the American people. I am going to keep that commitment. I think the American people have every reason to expect him to keep his commitment."

It's McCain's latest move to look past one-time Democratic front-runner Hillary Clinton and directly challenge Obama, as the Illinois senator takes the lead in the delegate count. McCain, the presumptive GOP nominee, first called on Obama Thursday to stop hiding details on his congressional earmarks -- even though a report showed Clinton helped secure nearly four times as many earmarks for her state than Obama did last year.

Though McCain said Thursday that he's not counting Clinton out, by raising the public financing issue he's already treating Obama like the party nominee.

The New York Times reported Friday that McCain's campaign was moving to press the public financing matter with Obama. The article said Obama issued a challenge a year ago to only use the public money in a general election, and that McCain was the only candidate who accepted the challenge.

But since then Obama's fundraising has skyrocketed, meaning he could have far more money by just using his own funds in a general election.

Obama got a big boost Friday with the endorsement of the 1.9 million-member Service Employees International Union, and a day earlier took the backing of the United Food and Commercial Workers Union, which is 1.3 million strong.

But the latest delegate tallies show the race is still close, with Obama at 1,276 and Clinton at 1,220. Clinton is planning for an impressive showing at the Ohio and Texas primaries on March 4.

Obama said Friday in Milwaukee he's not entertaining hypothetical scenarios yet.

"If I am the nominee I will make sure our people talk to John McCain's people to find out if we are willing to abide by the same rules and regulations with respect to the general election going forward," he said. "It would be presumptuous of me to start saying now that I am locking in to something when I don't even know if the other side will agree to it. And I'm not the nominee yet."

McCain would be the obvious beneficiary if they both take the federal money because they would have to return money already raised toward the general election; Obama has raised $6.1 million for the general, nearly three times as much as McCain's $2.2 million.

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."

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 said Friday if Obama "goes back on his commitment," then he would "rethink" whether he would use public financing as well.

Click here to read the article about public financing in The New York Times.

FOX News' Mosheh Oinounou and Bonney Kapp and The Associated Press contributed to this report.