Story highlights Concerns included investigation of Christie's travel expenditures while U.S. attorney.

Revelation included in new book by reporters on presidential campaign, "Double Down"

Christie faces New Jersey voters next week in gubernatorial reelection bid

Book also says Obama campaign conducted focus groups, polling about replacing Biden with Clinton

Ethics troubles complicated New Jersey Gov. Chris Christie's chances of becoming Mitt Romney's running mate last year, The New York Times reports is in an upcoming chronicle of the 2012 presidential campaign.

The Times reported that too many questions remained after Christie completed the vetting process and Romney had lingering concerns.

The details come four days before New Jersey voters head to the polls to determine if Christie gets a second term.

During a campaign stop Friday, he dismissed the report and told CNN that the authors are "just trying to sell books."

Christie's challenges were first reported Thursday night by Times national political correspondent Johnathan Martin and based on details of the book, "Double Down: Game Change 2012."

The highly anticipated book about the presidential election by Mark Halperin and John Heilemann, is expected to be a juicy tell-all about the inner-workings of the presidential campaign.

Those concerns included a Justice Department investigation of Christie's expenditures while he was a U.S. Attorney in New Jersey.

A 2010 report by the department's Inspector General concluded Christie — among five top prosecutors investigated -- was the "the U.S. attorney who most often exceeded the government (travel expense) rate without adequate justification."

He also offered "insufficient, inaccurate or no justification" for most of those costs, and refused to meet personally with investigators about the discrepancies.

Christie was not identified by name in the government report, but government sources have said he was "U.S. Attorney C," CNN confirmed.

Romney adviser Beth Myers, who ran Romney's vice presidential search committee, told CNN that "Governor Christie complied fully with the Romney campaign's request for documents in a timely manner, including a complete medical report from his internist and cardiologist."

Romney ultimately chose Rep. Paul Ryan of Wisconsin as his running mate.

Biden vs. Clinton

President Barack Obama's closest advisers may have secretly considered replacing Vice President Joe Biden with Hillary Clinton on the 2012 ticket, but White House spokesman Jay Carney said Friday that it was never a remote possibility.

"Campaigns and pollsters test a lot of things," he told reporters at the daily White House briefing. "What I can tell you without a doubt is that the President never considered that and if anyone brought that to him, he would have laughed it out of the room, would never take it seriously."

Carney earlier told CNN's "New Day" that he knew "for a fact" Obama never considered it, later saying the idea was never brought to him.

He answered affirmatively when asked if Obama thought Biden would be a strong chief executive, saying that he had been an asset to the President in two campaigns and throughout the administration.

"Sure. I think the President picked Joe Biden as his running mate for the right reason, which is, if necessary, the vice president could be president. That's the first item on your checklist when you're picking your running mate," Carney said at the White House.

Asked about sizing up the potential Republican presidential field, Carney replied, "I think they all ought to run. It would be awesome."

Bill Daley, who was then Obama's chief of staff, said the swap wasn't necessary.

No one "thought that that was a good idea or needed to be done or should be done," Daley said on "CBS This Morning."

The Times obtained a copy of the book and reported that the President's top aides conducted "extensive group-sessions and polling in late 2011" to gauge whether dumping Biden could help bolster Obama's waning re-election hopes.

The book says Daley spearheaded the effort, despite their "close personal rapport," before ultimately deciding against the move when data showed that adding Clinton to the ticket wouldn't "materially improve Obama's odds."

Daley pushed back on the notion.

"Not for a moment was there a serious discussion or a belief that Joe Biden should be replaced, period," said Daley, a CBS contributor.

A difficult political year

Like Carney said, Daley admitted that it was something he considered and that polling and research were done to explore the possibility because it was his job to "look outside the box."

"But in 2011," Daley said, "it was a very difficult political year, and so my sense was we ought to look at everything here because this is a very -- it was a very difficult period, politically."

Polls conducted by CNN at the time showed that the President was struggling politically a year out from Election Day 2012. In the late summer and fall of 2011, Obama's approval ratings hovered around 45%, a 10-point drop from January of that year.

There was much speculation over whether Biden would be on a second-term ticket in late 2011 and early 2012.

Wolf Blitzer wrote on CNN.com in October 2011 that White House sources told him that the President was "very high" on Biden and "deeply appreciates his advice."

He said he asked Clinton, then Secretary of State, if it was something she would consider but she said, "No."

Obama's former deputy press secretary, Bill Burton, told CNN's Erin Burnett in January 2012, "Even privately, President Obama would tell you that one of the best decisions he's ever made in politics was putting Vice President Biden on his ticket."

"He's not leaving the ticket," Burton added.

According to Jonathan Martin, the Times' national political correspondent, the book provides a thorough account of the effort by senior officials inside the campaign and the White House, namely Daley, to measure what effect swapping Clinton for Biden would have on the polls.

The potential switch was a closely guarded secret within the Chicago campaign infrastructure and inside the Oval Office.

Only half a dozen of the President's closest advisers -- including Daley, former Obama campaign chief Jim Messina, and former White House senior advisers David Axelrod and David Plouffe -- knew the change was under consideration.

Messina, Plouffe and Axelrod took to Twitter to push back on the report.

Plouffe denied the White House or the campaign ever contemplated the swap.

"Never any consideration of (Biden-for-Clinton) switch," Plouffe wrote in a tweet.

The move was "not ever entertained by the only person who mattered. Or most of us. Back to Halloween," he added.

Axelrod tweeted: "VP swap never in play. Biden's taken on many tough assignments. He's been loyal friend and wise counselor. POTUS lucky to have him," Axelrod wrote Friday.

And Messina said: "Let me be clear: VP-HRC switch never considered by POTUS or anyone who mattered in the campaign. #sillyseason"

A significant investment

But Martin pointed out that it was, in fact, considered.

Martin told CNN's Anderson Cooper the re-election campaign made a significant investment in finding out whether the move would pay dividends at the polls.

"Campaigns don't spend the kind of money on polling and focus groups unless they're seriously considering something," he said on CNN's "AC360."

He said on the program that he asked Daley whether his then-boss knew about the potential shuffle. While Daley said he doesn't think the President "was aware" of the potential change, the former chief of staff admitted that it's "possible" Obama knew.

Martin added that "Double Down" does not definitively answer whether the political probing reached Obama's desk.

As to whether Clinton knew, Daley said on CBS that if she did know, she "didn't find out from me."