The billionaire former CEO, who was a lifelong Democrat before he switched to the GOP for his first mayoral run, said the change in voter registration does not mean he is running for president.
"Although my plans for the future haven't changed, I believe this brings my affiliation into alignment with how I have led and will continue to lead our city," he said in a statement.
Despite his coyness about his aspirations, the mayor's decision to switch stokes speculation that he will pursue the White House, challenging the Democratic and Republican nominees with a legitimate and well-financed third-party bid.
Bloomberg has an estimated worth of more than $5 billion and easily could underwrite a White House run, much like Texas businessman Ross Perot in 1992. Bloomberg spent more than $155 million for his two mayoral campaigns, including $85 million when he won his second term in 2005.
The 65-year-old mayor has fueled the presidential buzz with increasing out-of-state travel, including New Hampshire last weekend; a greater focus on national issues and repeated criticism of the partisan politics that dominate Washington.
"The politics of partisanship and the resulting inaction and excuses have paralyzed decision-making, primarily at the federal level, and the big issues of the day are not being addressed, leaving our future in jeopardy," he said in a speech Monday at the start of a University of Southern California conference about the advantages of nonpartisan governing.
A Bloomberg entry would roil the already volatile and wide-open race to succeed President Bush.
"If he runs, this guarantees a Republican will be the next president of the United States. The Democrats have to be shaking in their boots," said Greg Strimple, a Republican strategist in New York who is unaligned in the race.
The belief among some operatives is that Bloomberg's moderate positions would siphon votes from the Democratic nominee. Others say it's not clear and his impact would depend on the nominees.
Former Democratic Party Chairman Donald Fowler said Bloomberg would be "a disturbing factor to both parties," but the mayor would probably draw more Republican votes simply because "Republicans are more disenchanted than Democrats."
"Democrats are pretty happy with their candidates," Fowler said. "The Republicans are absolutely in disarray."
He called Bloomberg "an exceptionally capable guy" who is "hard-nosed and accomplished," but argued that the obstacles for a third-party candidate are so daunting that it would be nearly impossible for Bloomberg to win.
Most polls find Bloomberg drawing votes from Republicans.
"He could have a significant impact on the campaign," said independent pollster Scott Rasmussen. "Nationally there's a significant segment of the electorate that would give serious consideration to Bloomberg as a candidate."
Strategists say he could mount a third-party campaign by stressing that he is a two-term mayor in a Democratic city and that he built his reputation as a political independent, social moderate and fiscal conservative.
Throughout his 5 ½ years as mayor, Bloomberg has often been at odds with his party and Bush. He supports gay marriage, abortion rights, gun control and stem cell research, and raised property taxes to help solve a fiscal crisis after the Sept. 11 terrorist attacks.
But he never seemed willing to part with the GOP completely, raising money for the 2004 presidential convention and contributing to Bush and other Republican candidates.
Just last year, he told a group of Manhattan Republicans about his run for mayor: "I couldn't be prouder to run on the Republican ticket and be a Republican."
On most occasions, Bloomberg has rolled his eyes at the suggestion that he might one day be a presidential contestant. But during a holiday party with City Hall staffers last December he performed a Bruce Springsteen rendition of "Born to Run."
Appearing Monday at Google Inc.'s California campus, Bloomberg teased questioners about a presidential bid, refusing to rule out the prospect but repeating that he plans to serve out his term through 2009. And he didn't debunk a report that he talked about an independent presidential bid with former Sen. David Boren, D-Okla.
Asked about a hypothetical independent candidate entering the race, Bloomberg launched a broad critique of the Bush administration and Congress and lamented the presidential debates to date.
"I think the country is in trouble," Bloomberg said, citing the war in Iraq and America's declining standing globally.
"Our reputation has been hurt very badly in the last few years," he said. "We've had a go-it-alone mentality in a world where, because of communications and transportation, you should be going exactly in the other direction."
His entry into the campaign would give the presidential contest a decidedly New York flavor, with Hillary Rodham Clinton, the New York senator on the Democratic side, and former New York mayor Rudy Giuliani on the Republican.