Try as he may, John McCain just can't seem to lock down Barack Obama on a public financing pledge.

Though aides to McCain say the presumptive GOP nominee will likely accept $84 million in public funds for the general election race, the frontrunning Democrat appeared to strengthen his argument against taking the money Friday in Indianapolis.

"I think that it is creaky," Obama said of the program financed by $3 checkoffs in tax returns.

"The amount of money raised through the public financing system may be substantially lower than the amount of money that can be raised over the Internet, which presents candidates then with some pretty tough decisions in terms of how they want to move forward if they want to compete in as many states as possible."

Only a few days ago, Obama likened his own fundraising to a "parallel public financing system," a remark that set off speculation that if he wins the Democratic nomination, he will be the first presidential candidate in three decades to forgo federal funds.

Obama has been understandably non-committal on the subject.

While McCain had a good March, raising about $15 million and bringing his total to $75 million, Obama had a much better month.

He raised $40 million, for a total of $234 million. Hillary Clinton raised $20 million for a total of $175 million.

Because of his fund-raising success, Obama is hesitant about committing to a limit on his war chest.

He said Friday he wasn't trying to "send a signal" with his "parallel public financing" remark, but that "what I was trying to suggest is that through the Internet and the enthusiasm of this campaign, we've created a model for being able to compete at the highest levels of politics without being dependent on big-monied interests."

He said his official "position" on the program is that "I would like to see a system preserved, and I intend, if I am the nominee, to have conversations with Senator McCain about how to move forward in a way that doesn't allow third parties to overwhelm the system."

That still leaves the door way open for shunning public funds -- a move the McCain campaign says would flout what they see as a pledge Obama made last year to accept public financing if he won the nomination.

McCain put the onus on Obama Friday to decide whether or not they take the public financing.

"Our commitment was that if the Democrat nominee would take public financing, we hold to that commitment ... But if Senator Obama is the nominee and decides not to take public financing, then obviously we have to evaluate our options on it," he said in Dallas. "It's possible that I still will and it's possible that I won't."

The Arizona senator is already taking steps to do so, returning money he has raised that was designated for the general election.

McCain spokesman Tucker Bounds tried to draw the line with Obama Friday.

"He has clearly committed to public financing in the general election should he win his party's nomination, and we expect him to keep his word," he said. "Any hedging or clever language from Senator Obama seems more like something you would read in a predatory home-loan, not the ‘Audacity of Hope'."

Obama noted that participation in the $3 checkoff had declined, reducing the amount of money in the fund.

Obama himself did not check off the $3 designation in his 2005 and 2006 tax returns. Obama, who had checked off the box in previous returns, said it was an oversight -- but his campaign later said that an amended 2006 tax return did assign the $3 to public financing.

FOX News' Molly Henneberg and Aaron Bruns and The Associated Press contributed to this report.