Obama's advisers considered replacing Biden with Clinton, according to book William M. Daley, the White House chief of staff at the time, was the most forceful advocate of exploring the switch, which the authors write was studied by "the top echelon of Obamaworld" and kept secret from the vice president. But the team concluded that adding Clinton "wouldn't materially improve" Obama's chances of winning a second term. Obama adviser David Plouffe, who the book says was part of the small group, tweeted Thursday that the idea of swapping in Clinton was "not even entertained." The new account paints the clearest portrait yet of the lengths to which Obama's advisers went to explore replacing Biden, a move that had been rumored but never spelled out in detail. The book, billed as a sequel to "Game Change," Halperin and Heilemann's account of the 2008 election, goes on sale Tuesday. The book's revelations were first reported Thursday evening by the New York Times. The Washington Post independently obtained a copy. The book is a narrative reconstruction of the behind-the-scenes machinations of the campaigns of Obama and Republican nominee Mitt Romney. It details Romney's search for a vice-presidential nominee. "Project Goldfish," as his vetting team called its operation, was so secretive that researchers referred to the five finalists by aquatic names - New Jersey Gov. Chris Christie (Pufferfish), former Minnesota governor Tim Pawlenty (Lakefish), Sen. Rob Portman of Ohio (Filet-O-Fish), Sen. Marco Rubio of Florida (Pescado) and Rep. Paul Ryan of Wisconsin (Fishconsin). Romney initially crossed Christie off his short list. The governor's vetting file was incomplete, and Romney had been bothered by Christie's propensity to show up late at campaign events and by his lack of physical fitness, the book says. "Romney marveled at Christie's girth, his difficulties in making his way down the narrow aisle of the campaign bus," the authors write. "Watching a video of Christie without his suit jacket on, Romney cackled to his aides, "Guys! Look at that!"" But Romney reconsidered Christie at the strenuous urging of two top campaign strategists, Stuart Stevens and Russ Schriefer. An 11-day crash vetting left unknowns, however. According to a memo published in the book, the unanswered questions included those about a defamation lawsuit from earlier in Christie's career; a Securities and Exchange Commission settlement involving Christie's brother, Todd; the names and documented status of Christie's household employees; his clients from his time as a securities industry lobbyist; and his medical history.