Mayor Bloomberg could wait until as late as April 2008 to add his name to the list of presidential contenders.

A decision by next spring would give Mr. Bloomberg enough time to meet the nation’s first ballot deadline, which is in Texas in May, a leading ballot expert said.

“I would say that he could afford to do nothing until April 2008,” the editor of Ballot Access News, Richard Winger, said.

Mr. Winger, whose California-based newsletter tracks ballot issues nationwide, noted, however, that the threshold is higher for independents or third-party candidates to get their names on the ballot.

He said state laws require independents or third-party candidates to collect a minimum of 700,000 valid signatures to get on the ballot in all 50 states and Washington, D.C, while Democrats and Republicans need 25,000 and 20,000, respectively.

“Whereas Giuliani will only need to get 20,000 signatures, Bloomberg would require 700,000. That’s an extreme difference,” Mr. Winger said. “But the advantage of being an independent is that you can sit around and do nothing for quite a long time.”

In Texas, the first state Mr. Bloomberg will have to deal with if he runs, an independent candidate is required to collect approximately 75,000 signatures from voters who have not cast ballots in the primary or nearly 44,000 to create a new party line.

Mr. Bloomberg has publicly denied that he is going to run for president, but even those who have worked closely with him say there is a strong chance he’ll run as a third-party candidate if the nominees selected by the Democrats and Republicans leave an opening.

Earlier this month, a former advisor to Mr. Bloomberg, Ester Fuchs, told a panel at Fordham University that the mayor is 80% likely to run if the Democrat and Republican hail from the “extreme wings” of their parties.

If Mr. Bloomberg does run he could end up finding his match in Unity08, which is trying to draft a bi-partisan ticket to compete with the nominees from the Republican and Democratic parties.

One of the founding members of Unity08, Douglas Bailey, a co-founder of the Hotline political newsletter and a former Republican consultant, said the delegates it signs up (it aims to attract 10 million) will choose its candidate.

Mr. Bailey said the party — which is planning to nominate a ticket that would include some combination of a Republican, a Democrat, and a independent — has already talked to between 60 and 70 candidates or their staffs. He would not say who Unity08 has met with, but said they have talked to leading candidates from both the Democratic and Republican field. He also praised Mr. Bloomberg and said he is “the kind of person” that the group hoped would seek its nomination.

Mr. Bailey also noted that in 1992 independent candidate Ross Perot didn’t start his signature collecting operation until the spring. Mr. Perot won about 19% of the popular vote, but did not win any electoral votes. He did get on the ballot in all 50 states.

“The fact of the matter is, it can happen,” said Mr. Bailey, whose group plans to nominate a candidate during an on-line political convention in June 2008.

Getting on the ballot is complicated and laborious process because every state has its own deadline for submitting signed petitions, its own rules for the number of required signatures, and its own guidelines about who can collect and sign petitions.

Mr. Winger said that 39 states allow a new party, like Unity08, to qualify for the ballot before it names a candidate.

If Unity08 successfully establishes itself, which is still a question mark, “they’ll relieve the ballot access burden for whoever their presidential candidate is and make their nomination extremely attractive,” Mr. Winger said.

Mr. Winger said that while independent candidates have a May deadline to submit petitions for the ballot in Texas, most states don’t require petitions until July, August, or September. Texas is also considering moving up its primary, which could mean that independents would have to submit petitions before May 2008.

A professor at Loyola Law School in Los Angles, Richard Hasen, said third-party candidate can expect resistance from whichever party they pose the greater danger too. In the past Democrats have relentlessly tried to knock Ralph Nader off the ballot for fear he would eat into their votes.

“The parties are not afraid to litigate election law issues,” Mr. Hasen said. He noted that the outcome of the election could have been different if the first President Bush launched a campaign to knock Mr. Perot off the ballot in 1992.

“I’m not sure who Bloomberg would benefit,” he added. “It remains to be seen who would be fighting to keep him off the ballot.”

A spokesman for Mr. Bloomberg, Stuart Loeser, said: “It’s an interesting Social Studies lesson, but from our perspective it’s only academic, since Mayor Bloomberg is not running for president.”