Barack Obama wants to campaign on his ideals, but he also wants to win. If you want to gauge the gap between his principles and his pragmatics, follow the money.

Ideally, Obama would be honoring a promise he made in 2007. He vowed that, if he became the Democratic nominee, he would run his autumn race in accordance with the federal reform rules that have guided presidential elections since 1976.

He said he'd stop raising money from private donors, and instead finance his race with $85 million from the federal kitty, as long as his Republican opponent did the same. John McCain agreed to those terms 15 months ago.

That pledge has since become highly inconvenient, which is why Obama repeatedly signaled this spring he intended to weasel out of it. Last Thursday, he did. He told his supporters in an online message that he intends to build "the first general-election campaign that's truly funded by the American people," which is high-road spin for saying, "I intend to become the first candidate to successfully privatize my autumn campaign since Richard Nixon did it in 1972."

The pragmatics seem obvious: Why should Obama agree to a level playing field — $85 million for him, the same for McCain — when he can simply re-stoke his private fund-raising juggernaut for the final round and tilt the field in his favor?

Compared with the private-donor money that Obama can raise on his own for the fall, $85 million in taxpayers' money is chump change. He has already racked up roughly $300 million for the primary season (a record haul for a candidate), and party insiders say he could raise $500 million more for the 10-week sprint to Election Day.

McCain, by contrast, cannot touch that kind of money. Republicans are demoralized, and a lot of conservative donors don't trust him.

He's stuck with two choices, neither attractive: He can take the public money (and be overwhelmed by Obama's private money), or he can follow Obama's lead and refuse the public money (and wind up spending his time going to fund-raisers — unlike Obama, who garners much of his money on the Internet).

There's a downside to Obama's decision, at least in theory. He has indeed gone back on his word. He made a promise, and he broke it. He has styled himself a reformer, yet he has made a pragmatic decision in the style of a typical politician. The McCain people started hammering on that theme when Obama was first signaling his intentions, and now they're trying to frame the money flip-flop as a character issue.

On the other hand, perhaps Obama is simply doing what it takes to win — as Republicans have long been known to do. He figures that if he gets some grief for his decision, so be it, because the nuts-and-bolts benefits of outspending McCain by as much as 5-1 are so obvious.

He'll have the money to expand the electoral map. He can invest heavily in states that Democratic candidates typically short-change or ignore (Virginia, North Carolina, Georgia, Nevada, Colorado, Montana), thereby forcing McCain to spend precious money on the defense of red-state turf and making it all the more difficult for McCain to compete effectively in traditional battlegrounds such as Pennsylvania and Michigan.

Obama is aware some Democrats still question whether he's a fighter; Hillary Clinton's pitch was that Obama was too much of a dreamer to win. So his decision to defy public financing is aimed at them as well.

He's now arguing that he needs maximum bucks just to fight the slew of well-financed independent groups that intend to attack him this fall. Indeed, McCain made it clear earlier this month that he does not intend to "referee" the behavior of those groups, and Obama is now seizing on that.

But, all pragmatics aside, Obama the clever rhetorician has also crafted a high-minded rationale for his pledge break. On Thursday, he described his private fund-raising machine as "a new kind of politics ... ordinary people coming together." Lest people think he is defying public financing to be powered by fat cats, he cited the unprecedented participation of small donors.

He's right. Nearly 50 percent of his $300 million has come from people giving $200 or less. In other words, his grassroots credentials will shield him from the full force of the flip-flop charge.

I doubt Obama will take a big hit for his broken promise. Reform groups, good-government citizens, and editorial writers will complain, but most Americans couldn't care less about the money issue. Last year, when Gallup asked whether candidates should take public financing, or simply use the money they raise on their own, 39 percent of the respondents chose the former and 56 percent the latter.

Nor do most Americans want to finance the presidential campaigns. When they do their tax returns, only 10 percent typically agree to check the box that sets aside $3 for the program. The '70s Watergate reforms now seem as dated as disco.

So maybe Obama's halo has been dented a bit, but most Democratic voters are in no mood to indulge idealism. In their hunger for victory, they're willing to swallow a lot.

Dick Polman is a columnist for the Philadelphia Inquirer. Reach him atdpolman@phillynews.com.