NEW YORK — Former President Clinton warned Monday that a Mitt Romney presidency would be “calamitous” for the nation and the world, going further than even President Obama in depicting the consequences of a return to Republican rule of the White House.

With Obama standing thoughtfully to one side, Clinton slammed Romney by name, an apparent rebuttal to his own comments last week that were widely seen as flattering to Romney’s background in business.

Clinton said Obama had earned a second term because of his steering of the economy through a “miserable situation” and “the alternative would be, in my opinion, calamitous for our country and the world.”

Clinton’s take came as he helped raise at least $3.6 million for Obama at three New York fundraisers. The two have patched over a personal rift from the 2008 campaign when Obama defeated Hillary Rodham Clinton in a bitter Democratic primary.

But Clinton caused some heartburn in Obama’s campaign last week by remarking that Romney had a “sterling” business record — an assertion that undercut Democrats’ criticism of Romney’s decisions at the private equity firm Bain Capital.

Clinton also said at the fundraiser that Republicans and Romney have adopted Europe’s economic policies. “Who would have ever thought that the Republicans who made a living for decades deriding Old Europe would embrace their economic policies?” he said.

For his part, Obama said the economy had been difficult for so many voters that some could reach the point that “you’re willing to try just about anything, even if you’ve seen it before.”

Obama told donors that he was seeking re-election against a different Republican Party. “They have run from a preference for market-based solutions to an absolutism when it comes to the marketplace, a belief that all regulations are bad, that government has no role to play,” he said.