A political strategist and pollster who has advised President Clinton and Mayor Bloomberg is arguing that America could elect a third-party candidate to be president in 2008, and discloses how such a campaign might be pulled off in a new book that seems designed to convince Mr. Bloomberg to launch a White House bid.

In “Declaring Independence: The Beginning of the End of the Two-Party System,” Douglas Schoen constructs an electoral road map for an independent candidate and promotes Mr. Bloomberg as a possible contender with the potential to mount a serious challenge to the Democratic and Republican candidates.

Mr. Schoen, who is close to Mr. Bloomberg and worked on his mayoral campaigns, wrote that a third-party ticket with Mr. Bloomberg for president and Senator Hagel of Nebraska for vice president would be powerful.

It “would have enormous appeal for disaffected Democrats and independents as well as moderate Republicans,” he wrote, according to an advance uncorrected copy of the book obtained by The New York Sun. “It would provide a broad geographic appeal — an urban and a rural candidate — and both politicians’ strengths would complement each other nicely.”

Messrs. Bloomberg and Hagel, a Republican, are attending a bipartisan political meeting in Oklahoma on Sunday and Monday with officials from both parties that could set the stage for Mr. Bloomberg’s candidacy. The meeting, coupled with the mayor’s critique of the major party candidates earlier this week, is stoking speculation that he will run.

Although Mr. Bloomberg denies any plans to seek the presidency, Mr. Schoen writes in the book, due out from Random House in February, that the mayor might reconsider if he believed he had a real chance of winning. Mr. Bloomberg hosted a party for Mr. Schoen in his Upper East Side townhouse after he wrote an earlier book, “The Power of the Vote: Electing Presidents, Overthrowing Dictators, and Promoting Democracy Around the World.”

Mr. Schoen, a campaign consultant who spent more than 30 years at the firm of Penn, Schoen & Berland Associates, would be a likely addition to a Bloomberg presidential campaign, but a non-compete clause in his contract with the firm could prevent him from accepting a position. The Politico, however, has reported that the substance of the clause may be in dispute, indicating it might be possible for Mr. Schoen to work for the mayor if he ran. Mrs. Clinton’s chief strategist is Mark Penn, who is president of Penn, Schoen & Berland Associates.

Mr. Schoen said in a telephone interview yesterday that he has no expectation of getting involved with Mr. Bloomberg’s campaign, should Mr. Bloomberg launch one. He refused to say in the interview whether Mr. Bloomberg could win the presidency.

“I argue in the book that we are at a unique time in American political history, and I believe there is a compelling opportunity for a third-party candidate,” he said.

Mr. Schoen said he thinks Mr. Bloomberg will revisit the question of whether to run once the nominees have been selected and the mayor’s chief political strategist, Kevin Sheekey, is finished with his analysis of the field.

Mr. Schoen said he did not speak to Mr. Bloomberg about the book, but did speak at length with Mr. Sheekey, whom he called a friend and colleague with great political insight.

Mr. Sheekey has said he will urge the mayor to run if 70% of Americans are convinced the country is on the wrong track and 40% have an unfavorable view of the Democratic and Republican candidates.

In the chapter on crafting an electoral strategy, Mr. Schoen looks to Ross Perot’s performance during the 1992 presidential race as an indicator of how well a third-party candidate could do this year, noting that in a three-way race, a third-party hopeful would need to win only 34% of the vote to win a state’s electoral votes.

“If Perot, with all his personality quirks and the absence of a political resume, could still score close to 20 percent of the vote in a national election, a better qualified candidate could easily better that total,” Mr. Schoen wrote.

Mr. Perot won more than 20% of the vote in California, Washington, Wyoming, Idaho, Alaska, Oregon, Utah, Colorado, Arizona, Nevada, New Hampshire, Vermont, Connecticut, Rhode Island, Maine, and Massachusetts. Winning votes in the South, the Midwest, the states bordering the former Confederacy, and in New York, Pennsylvania, and New Jersey will be a challenge for third-party candidates, Mr. Schoen wrote.

He also provides a rundown of various scenarios that could play out if no single candidate wins a majority of the Electoral College in a three-way race, positing that two candidates could pool their votes in a power-sharing agreement that would lead to America’s first coalition government.

In one scenario, Mr. Schoen explains that an independent candidate could become president even without the 270 electoral votes required. A Republican in last place could decide to support an independent candidate for president in return for key cabinet positions, he writes.

If the candidates didn’t reach an agreement, the outcome of the election would be decided in the House of Representatives, where more bargaining would occur, or in the Senate if there was gridlock in the House. If the Senate couldn’t reach an agreement, the Speaker of the House, Nancy Pelosi, could become the next president, Mr. Schoen wrote.

“The possibilities for national horse trading over the White House and the terms, platform, and makeup of the next government are limitless,” Mr. Schoen writes.

“While it is not too much to expect a third-party candidate to upend the political system and win a national election, it’s also very likely that a third-party candidate could in a multitude of ways impact the nation’s ultimate choice of the president, or at least the national agenda,” he added.