The multibillionaire businessman and mayor of New York, Michael Bloomberg, is pushing ahead with plans to throw himself into the US presidential contest as an independent candidate this spring.

Bloomberg, who has a personal fortune that would allow him to mount the most expensive campaign in US political history, could seriously damage the chances of the Democratic candidate.

Bloomberg is waiting to see the outcome of the Republican and Democratic races before deciding whether to announce his candidacy in March. But his staff want him to run and there is a blueprint drawn up for contesting the 50 states.

Journalists who cover Bloomberg, a member of the Democratic party before switching to the Republicans - from which he has now also resigned - say that if Barack Obama is the Democratic choice, he will not stand. The same is true if John McCain, the most moderate of the Republican field, wins his party's nomination.

Bloomberg, while denying he intends to run, spent this week raising his political profile. He used the annual state of the city address on Thursday to make the usual city-specific announcements, such as 500 more beds for the homeless, but also to talk about national issues, such as immigration, tax relief and crime.

He is travelling to other states to raise his political profile and to engage in national policy debates. Yesterday he visited Texas to make a health announcement with the Tour de France hero and cancer survivor Lance Armstrong. Today, he will be appearing in California for a meeting with the governor, Arnold Schwarzenegger, to discuss infrastructure.

Although it has been confirmed he is paying out millions to pollsters to test support in the 50 states for a presidential run, he snapped at a reporter who questioned him this week about whether he would be a candidate. "Miss, no matter how many times you ask the question, I'm not a candidate. That's the answer. I can't go into nitpicking. This is ridiculous," he said.

Independents have never won in recent US political history but they have influenced the contests. Ralph Nader, who ran as an independent in 2000, is often blamed for taking votes from Al Gore and allowing victory to George Bush.

Ross Perot, who stood in 1992, spending $65m, had a disastrous campaign but still managed to win 18.9% of the popular vote.

Larry Sabato, professor of politics at the University of Virginia, said a Bloomberg run could have enormous influence.

"Normally, independents do not have much impact but this one is worth between $11bn and $20bn and he would be willing to spend at least $1bn [about £510m] on this. If he spends a billion, he would have outspent the Democratic and Republican candidates combined. It is bound to have an impact."

Stephen Hess, a political analyst at the Brookings Institution who served in the Eisenhower and Nixon administrations and was an adviser to presidents Ford and Carter, said yesterday that he would be surprised if he stood, given the risk that he would prevent a Democrat taking the White House.

"I know he is considering the race. I think he has been a great mayor and philanthropist and I do not know why he would want to go down in history as the Great Spoiler," Hess said.