NEW YORK — President Barack Obama and Mitt Romney have courted his endorsement covetously, with a White House lunch invitation here (Obama) and a pilgrimage to the Upper East Side there (Romney), through intensive back-channel outreach and effusive public praise.

But sitting in the elegant town house in Manhattan that houses his private foundation two and a half weeks before Election Day, Mayor Michael R. Bloomberg, who has endorsed neither, had sobering words for both.

For Romney: "I do think that Romney’s business experience would be valuable, but I don’t know that running Bain Capital gives you the experience to run the country."

For Obama: "This business of ‘Well, they can afford it; they should pay their fair share?’ Who are you to say ‘Somebody else’s fair share?"’

For both: "Their economic plans are not real. I think that’s clear."

In one sense, Bloomberg was speaking for voters everywhere who feel frustrated over a presidential campaign that has been fought in sound bites and bromides as problems at home and abroad multiply.

But he was also stepping forward as the nation’s newest billionaire "super PAC" donor, with a vow to spend millions beyond this election year supporting candidates willing to do what he implied Obama and Romney were not: taking "leadership and standing up to do things that aren’t going to be popular."

And it was in that context that he offered his most expansive, and critical, take yet of the two men vying for the presidency, each of whom had at times held promise to practice Bloomberg’s brand of nonideological pragmatism only to become snared in partisanship.

His new group, Independence USA PAC, which was officially unveiled last week, has pledged to spend up to $15 million in the next two weeks on state, federal and local candidates whose views align with Bloomberg’s in support of gun control, same-sex marriage and overhauling public schools.

Calling that "getting your feet wet," he said he wanted to provide a financial bulwark for those who occupy his definition of the political center, as George Soros does for those on the left, and as Charles and David Koch do for those on the right.

In essence, though, Bloomberg is seeking to give the candidates a taste of the political freedom that he has enjoyed as a self-financed billionaire whose money helped him withstand the powerful opposition he faced because of his unpopular initiatives, including a smoking ban and an 18 percent raise in property taxes, to name just two.

While he had some praise for Obama and Romney, his central argument is that they are too hemmed in by partisan obligations and special-interest intimidation to tackle problems head on. (It was also hard not to see a glimpse of a self-made man who has had his own chronic yearnings for the presidency, sidelined by his conclusion that he could not win.)

"If you listen to what they say, they never get explicit," he said, leaning forward in a plush velvet lounge chair alongside the political aide who is running his new super PAC, Howard Wolfson.

Bloomberg endorsed George W. Bush in 2004, when the mayor was a Republican. In 2008, after changing his party registration to independent, he declined to make an endorsement.

His support is eagerly sought because of his unusual crossover appeal, especially to three key constituencies: moderate independents, Jewish voters and the Wall Street crowd that has shifted to Romney this year from Obama four years ago.

Though he is more likely to agree with Obama than with Romney — on same-sex marriage, climate change and abortion rights — he expressed disappointment with the president’s leadership.

"I am more in sync with President Obama’s views on social issues," he said. "I will say that I don’t see as much action as I would like, and it’s nice to be on the side that I think you should be on, but unless you do something, so what?"

Bloomberg was asked how he could make that criticism given the president’s announcement last spring that he supported same-sex marriage.

"Let’s get serious here: It was Joe Biden that forced that issue," he said, referring to the vice president’s surprise remarks backing same-sex nuptials that were credited with forcing Obama to follow. "Some people say he just goes off; I would say he’s a principled guy."

The White House has said that the president had always intended to announce his support for same-sex marriage and that Biden only forced him to do so slightly earlier than planned.

Bloomberg, who founded the global media giant Bloomberg LP, is worth an estimated $22 billion, according to Forbes, and he is occasionally mentioned as a future Treasury secretary. He described Romney as wrongheaded for his opposition to raising taxes as part of a budget deal.

"You cannot balance the budget without raising revenue and cutting expenses," Bloomberg said. "There is no reputable economist that remotely thinks you could do this without doing those two things — one of which is anathema to the Republicans, and one of which is anathema to the Democrats."

A real solution, he said, would be to allow the Bush-era tax cuts to expire as scheduled at the end of this year. Obama supports allowing them to expire for those with household incomes of more than $250,000, a delineation that Bloomberg described as unfair, arbitrary and fiscally irresponsible. Similarly, Bloomberg suggested that both men were engaging in election-year pandering in their tough talk on China. "I’m skeptical enough to think that a lot of this is just done by advisers," Bloomberg said. "‘I’m going to be dishonest to get elected because unless I get elected, I can’t be honest.’ It’s a little bit of that."

Most galling to him, however, is their lack of action on his marquee issue: gun control. "Romney passed a law, signed a law, in Massachusetts banning assault weapons, and today says, ‘No,"’ he said. "And Obama campaigned on introducing legislation to ban assault weapons, and then did absolutely nothing."

Bloomberg suggested they were cowed by the National Rifle Association, which has endorsed Romney.

The point of his super PAC, he said, was to provide "spine" for politicians under that sort of pressure, which is why, for instance, it is supporting the re-election of Rep. Robert J. Dold. A Republican from Illinois, Dold got a "D" from the National Rifle Association for backing some gun restrictions.

"You’re not going to beat the NRA overnight," Bloomberg said. "As you get going, people start realizing that there’s a sane group of people out there that want them to do intelligent things, and that that’s where their support is going to come from."

As for Romney and Obama, he said, he had not entirely ruled out endorsing one of them, unlikely as that may seem.

On Thursday evening, he saw both candidates at the annual Al Smith dinner, where they were feted, and where Obama poked fun at Bloomberg’s ban on extra-large sugary sodas. ("They all do that," Bloomberg said. "That’s easy.")

For a night, at least, they did not lobby for his endorsement. And he offered them advice for the roast. "I said to both of them, ‘Don’t be sensitive,"’ Bloomberg said. "‘They’ll kid me and they’ll kid you, and if they don’t mention your name, that’s worse."’