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.