In a powerful passage that can only hurt Romney, Bloomberg wrote on his website: "I believe Mitt Romney is a good and decent man and he would bring valuable business experience to the Oval Office.

"He understands that America was built on the promise of equal opportunity, not equal results. In the past he has also taken sensible positions on immigration, illegal guns, abortion rights and health care. But he has reversed course on all of them, and is even running against the health-care model he signed into law in Massachusetts.

"The devastation that Hurricane Sandy brought to New York City and much of the northeast - in lost lives, lost homes and lost business - brought the stakes of next Tuesday's presidential election into sharp relief.

"Our climate is changing. And while the increase in extreme weather we have experienced in New York City and around the world may or may not be the result of it, the risk that it might be - given this week's devastation - should compel all elected leaders to take immediate action."

Bloomberg's support comes at a time when the mayor's own profile has been boosted by his handling of the disaster.

It also comes after the Republican governor of New Jersey, Chris Christie, repeatedly praised Obama's leadership in responding to the crisis.

Republicans played down the significance of the Bloomberg endorsement. "It's not surprising to me. Bloomberg is a very liberal political figure," said George Pataki, the former Republican governor of New York.

After suspending his campaign for four days in the aftermath of superstorm Sandy, the president embarked on a punishing sprint across America.

He flew to Wisconsin for the start of 17 stops in eight swing states to Monday.