Mayor Bloomberg is leaving open the possibility that he will play a significant role in the 2008 presidential election, saying he would help a candidate who is willing to defy party ideology in favor of finding practical solutions to America’s problems win the White House.

In an op-ed article that puts to rest months of strong speculation that he would run for president as an independent candidate, Mr. Bloomberg says that while he has always said he is not running for president, the race is too important to sit on the sidelines.

“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,” he writes in today’s New York Times.

The statement indicates that Mr. Bloomberg, a billionaire who could have spent about $1 billion on a presidential campaign, may become one of the most important noncandidates for president.

“Over the past year, I have been working to raise issues that are important to New Yorkers and all Americans — and to speak plainly about common sense solutions. Some of these solutions have traditionally been seen as Republican, while others have been seen as Democratic. As a businessman, I never believed that either party had all the answers and, as mayor, I have seen just how true that is,” he wrote.

The speculation over his potential White House bid has been fueled by aides and supporters, and soared in the past year as Mr. Bloomberg crisscrossed the country speaking out on national issues, including the environment, immigration, and gun control.

Indicating that he was seriously considering mounting a presidential campaign, he had been paying for detailed nationwide voter analysis and had a system in place to get on the ballot in all 50 states, his colleagues have said.

Douglas Schoen, a political strategist who has advised Mr. Bloomberg and President Clinton and has been boosting the idea of a Bloomberg candidacy, said he considers the mayor’s op-ed a “call to arms.”

“This is Bloomberg saying, ‘Look. I’m going to be involved. I’m going to be influential. I may endorse a candidate. I may do an independent expenditure. I may rally my coalition of mayors on behalf of a candidate.’ And I see Bloomberg as one of the most important, if not the most important, noncandidate for president in America,” he said.

He said he considers Mr. Bloomberg a prospective vice president for the Democratic front-runner, Senator Obama, or the Republican front-runner, Senator McCain.

“There are very few people in America who could have that kind of decisive impact on a race like he could,” he said.

Mr. Schoen said he didn’t think Mr. Bloomberg had made a decision about which candidate he would support in the race, but noted that the mayor is close to Mr. McCain, has a good relationship with Mr. Obama, and has good feelings toward the other Democratic candidate, Senator Clinton.

“I believe that Mike Bloomberg is a patriot, and that if John McCain or Barack Obama asked him to serve, I think he’d have to take that very seriously,” he said.

Mr. Bloomberg has downplayed the idea that he would serve in the White House as a vice president, saying at one point that he is too old to work for someone else. Yesterday, in response to a question about a report in the Washington Post that said he was a popular candidate for the vice presidential seat among readers of the newspaper, he said the story “was very flattering.”

“But as you know, I have 673 days left in this job and I plan to serve them out,” he said. “I don’t think anybody is going to ask me to be vice president.”

Yesterday, before the Times released Mr. Bloomberg’s op-ed on its Web site, the mayor sought to dampen speculation that he would begin collecting signatures next week to secure a spot on the Texas ballot.

The day after the Texas primary, March 5, is the first day that independent candidates for president can begin circulating petitions in the state. A signature drive in the Lone Star State would have been the strongest sign yet of Mr. Bloomberg’s potential presidential plans.

He gave reporters at a press conference in Lower Manhattan his standard denial, saying he is not a candidate for president, but left the door open, if only ever so slightly, saying that his chief political aide and biggest presidential booster, Kevin Sheekey, might act on his own.

“Now what Mr. Sheekey is going to be doing — I can’t speak for him, control him, or anything else, but I think it is a fair assumption that we will not be doing a petition drive in Texas,” Mr. Bloomberg said.