NEW YORK -- After two years of playing coy about his presidential ambitions, Mayor Michael Bloomberg said Wednesday he will not run for president as an independent, declaring in a newspaper editorial that he might lend his support to a candidate who "takes an independent, nonpartisan approach."

The 66-year-old billionaire businessman, who aides had said was prepared to spend $1 billion on his own independent campaign, wrote in a New York Times editorial that he will be working to "steer the national conversation away from partisanship and toward unity; away from ideology and toward common sense; away from sound bites and toward substance."

"I listened carefully to those who encouraged me to run," Bloomberg wrote, "but I am not -- and will not be -- a candidate for president."

Bloomberg, who has almost two years left in his second term at City Hall, had publicly denied any interest in running for president since one of his political advisers first planted the seed more than two years ago.

But his denials grew weaker in recent months as aides and supporters quietly began laying the groundwork for a third-party campaign.

Among his biggest obstacles was getting on the ballot, a process that varies wildly from state to state and would have required him to obtain hundreds of thousands of signatures according to a timetable whose first key date is March 5.

Beginning next Wednesday, a Bloomberg campaign would have had to start gathering signatures to get on the ballot in Texas, which has one of the earliest deadlines.

Bloomberg's preparation for a presidential bid was extensive. The work included mass polling and nationwide data collection to determine his viability as a candidate, as well as detailed study and preparation for a state-by-state ballot access drive.

Aides and associates had said in recent days that Illinois Sen. Barack Obama's rise in the Democratic contest against New York Sen. Hillary Rodham Clinton was increasingly diminishing the chance that Bloomberg would run.

In the Times editorial, posted online Wednesday night, Bloomberg wrote that while he is not running, the race is too important for him to stay completely out.

"And so I have changed my mind in one area," he said. "If a candidate takes an independent, nonpartisan approach -- and embraces practical solutions that challenge party orthodoxy -- I'll join others in helping that candidate win the White House."

Putting his endorsement -- and wealth -- behind one of the candidates could make a significant difference. And Bloomberg, a Democrat-turned-Republican-turned-independent, has ties to Obama, Clinton and Republican Arizona Sen. John McCain.

Last November, Bloomberg met Obama for a highly publicized breakfast, and he has praised him for some of his positions, such as supporting merit pay for teachers.

He also has worked closely with Clinton, his home state senator, and has a friendly relationship with McCain.

Bloomberg's friend and political ally California Gov. Arnold Schwarzenegger has endorsed McCain, and the senator said during a Republican debate that Bloomberg had done "remarkable things" with the city's education system, the largest in the nation.

Click here to read Bloomberg editorial at New York Times Web site