SO HE’S NOT running for president. That’s what New York City Mayor Michael Bloomberg is saying now, even as his aides fan across America studying the possibility.

Whatever Bloomberg ultimately decides to do, his departure from the Republican Party is not insignificant. The day after announcing he had become an “independent,” Bloomberg launched a goodbye missile toward the administration of George W. Bush.

He said that “we’re in danger of losing our lead in many parts of science and medicine and education, economics.” That doesn’t leave us many things to be a leader of. Bloomberg blamed both parties, but we know who he means.

Can an independent candidate for president win? History says no, but recent polls suggest maybe.

More and more Americans think that the Democratic and Republican parties both stink. In a recent Rasmussen poll, nearly a third of Americans refused to identify with either party. Self-described independents ran about neck and neck with Republicans (31.5 percent) and not far behind Democrats (37.2 percent).

Before he bolted the GOP, romantics had paired Bloomberg with Nebraska Sen. Chuck Hagel or with California Gov. Arnold Schwarzenegger (who really can’t run because he’s foreign-born). The partners were all nominally Republicans, albeit mavericks, so — barring mass defections — these dream teams would have played within the framework of the two-party system.

Bloomberg’s politics were always pretty elastic. He started out as a Democrat but turned Republican to run for mayor. He’s governed as a friend of labor, education, the environment and surplus budgets.

As for third-party candidacies, they haven’t prevailed in presidential elections so far, but they can change the result. Neither Theodore Roosevelt in 1912 nor Ross Perot in 1992 won, but they did siphon enough support from a major party candidate to cause him to lose.

Bloomberg has two things going for him. One is money. Bloomberg is said to possess something north of $5 billion, which means he can finance his own campaign and hardly nick his estate. Some will argue that this means that he can’t be bought by lobbyists. The troubling question is whether we would be bought by him.

In terms of what’s good for America, Bloomberg has assets that are far more valuable: courage and the ability to govern.

Bloomberg is one of those elected officials who know what must be said and say it. When ideologues or special- interest groups throw a tantrum, they reply, “Frankly, my dear, I don’t give a damn.”

A year after 9/11, when Bloomberg raised property taxes rather than slash city services, the sclerotic right went into a war dance. Writing in the conservative City Journal, Steven Malanga accused Bloomberg of being “the defender of big government and the municipal workforce” and of committing “catastrophic errors” that would drive away businesses and rich people. The article was titled, “Bloomberg to City: Drop Dead.”

What the conservative wind-up dolls didn’t get is that taxes, wisely spent, can be an investment for the future. The devastating attacks had sent New York’s economy into a swoon, and sure, Bloomberg could have responded by laying off public workers who had kept the city’s soul together. He could have let things get shabbier.

Instead, Bloomberg decided to protect the New York City franchise — which is about urban amenity, not low taxes. As he said at the time, “We have to pick up however much garbage is out there.”

And so what subsequently happened? Major crime fell 30 percent, school-test scores rose, the welfare rolls declined, the budget went into surplus, and guess what: Bloomberg is now cutting some of those taxes he had to raise. Today the city has more rich people than you can stomach.

What’s Bloomberg really up to? His motives may be more modest than seeking the presidency — to remain politically potent now that he’s a lame-duck mayor (due to term limits). Bloomberg’s declaration of independence recharges the batteries on his spotlight, still burning bright after he shared a Time magazine cover with fellow innovator and problem-solver Schwarzenegger.

As the Bush administration enters its last 16 months of messing everything up, politicians who flash competence and think about the future offer hope to an increasingly frustrated public. Bloomberg has opened the window for a gasping populace.

Froma Harrop is a member of The Journal’s editorial board and a syndicated columnist.