Assuming he wins the Democratic Party’s presidential nomination, Sen. Barack Obama (Ill.) has a difficult decision to make about whether to accept public funds for the general election.

He still has to finish off Sen. Hillary Rodham Clinton (D-N.Y.), and this is no certainty, given how resilient she has proven when she’s in a hole, and how Obama’s image has slipped in the past month or so.

Nevertheless, the chances are that Obama will be the nominee, and Sen. John McCain (R-Ariz.) has been hammering the Illinois senator on the issue.

McCain repeatedly calls on Obama to keep his word, reminding voters that Obama agreed to accept public financing if the GOP nominee did. McCain has agreed to do so.

In an interview on “Fox News Sunday,” Obama pointed out that his campaign has mobilized small donors, adding that the average donation is $96.

Pressed on his earlier promise, Obama said he “would be very interested in pursuing public financing, because I think not every candidate is going to be able to do what I’ve done in this campaign, and I think it’s important to think about future campaigns.”

Obama is boxed in. If he doesn’t accept public financing, he will be criticized as a flip-flopper who abandons principle for political expediency. If he does embrace McCain’s challenge, Obama will be giving up his huge money advantage in the general election — and would certainly incur the wrath of the left, which attacked former Sen. John Edwards (D-N.C.) for taking that path.

Accepting public funds would be seen as noble by some people, but many Democrats are not thinking about “future campaigns.” They are desperately trying to win back the White House this year.

If Obama took public funds and ended up losing to McCain, he would be forever second-guessed.

One of Sen. John Kerry’s (D-Mass.) biggest regrets in his 2004 run for the White House was accepting public money. In retrospect, he recognized that spending limits handicapped his ability to respond to highly damaging advertisements casting doubt on his military record.

Obama suggested Sunday that he is not going to agree to any pact with McCain that allows the Republican National Committee or other right-wing organizations to spend vast amounts of money against him.

He is willing to sit down with McCain to talk about it. Both men have claimed the reform mantle, and there would be pressure on Obama to agree at that meeting to strike some sort of deal. Indeed, getting Obama to a sit-down with McCain would be viewed as a GOP victory.

Following his 10-point loss to Clinton in Pennsylvania, Obama is looking forward to the day he will make his decision on public financing. It will be, as Kerry will surely remind him, a big one.