Does Obama's Credibility Matter? Ask Jimmy Carter.

It is, without doubt, the meme of the day inside the Beltway, the latest explosion of conventional wisdom in Washington. Triggered by the botched launch of the Affordable Care Act and President Obama's retreat from an oft-repeated promise, the onslaught is underway. U.S. News, National Review, Commentary, MSNBC, Karl Rove, Peggy Noonan, and Investor's Business Daily have all warned of a looming "credibility gap" crippling the White House. The Washington Post posited the president's biggest challenge to be "restoring his credibility, not only on health care but also on his overall leadership." The indictment is a broad one, shifting in recent months as critics saw a president stripped of his credibility, first on Libya, then on fiscal issues, then on health care, and, most recently, on Iran.

Memories of Lyndon Johnson's Vietnam — the war that gave birth to the phrase "credibility gap" — and Richard Nixon's Watergate have been summoned, with their evocations of failed presidencies and hobbled White Houses. Regarding Obama, some Washington savants could rightly claim they were ahead of the crowd. Mother Jones warned in 2009 of an Obama credibility gap on Afghanistan. Columnist Bob Herbert in The New York Times spotted one in January 2010. The Root, an African-American publication, early on lamented a credibility gap for Obama on black issues. And Gerard Baker in The Australian didn't even wait for Obama's election, proclaiming a credibility gap in September 2008.

With the president publicly apologizing for his broken promise ("If you like your health care, you can keep it") and the administration frantically racing to fix the ACA website by the end of the month, it is hard to dispute that Obama's credibility is under siege. But nobody really knows how lasting or deep the damage will be.

It is logical to assume that damage has been done. 

Obama apologized again Thursday in his White House press conference, suggesting he was guilty of "two fumbles" in what he described as a big game. He acknowledged a hit to his credibility. But he pointedly added, "The game's not over" and promised to regain the trust of the American people. "I think it's legitimate for them to expect me to have to win back some credibility on this health care law in particular and on a whole range of these issues in general," he said. He added that the mistakes were his fault. "That's on me," he said flatly.

But he rejected any linkage between the "fumbles" on health care and the rest of his agenda, specifically insisting the political fallout from health care should not affect immigration reform.  Those who try to link the two, he said, are "looking for an excuse not to do the right thing on immigration reform."

Somewhat philosophically, he accepted the current criticism, saying, "There are going to be ups and downs during the course of my presidency. The latest indication of the current "down" came Wednesday when Gallup released new polling showing that only half the country viewed the president as "honest and trustworthy." Fifty percent now say that, down 5 points since September. This is usually the measure pollsters use to gauge credibility. At the least, it is a warning sign that the government shutdown and the flawed health care rollout have taken a toll on the president's personal image. But it is tougher to measure what this means for the rest of Obama's time in office, because neat logic doesn't always apply to the presidency. If it did, Bill Clinton would not have seen his approval ratings rise after he admitted he had lied to his wife and his Cabinet and was impeached for lying to a grand jury.

"Approval ratings go up and down," says Frank Newport, Gallup's editor in chief. "But basic faith in the person is more precious and a more fundamental judgment about a president. Once you lose that, it is harder to get it back." But, Newport tells National Journal, it would be a mistake to overreact to the current dip. "Fifty percent is still high. Compared to other presidents, 50 percent is not bad, when his overall approval ratings are down in the low 40s. In Clinton's last years in office, his honest and trustworthy was 20 points lower than that."

Inside the White House, top aides insist they take the threat to the president's credibility seriously. But they are focused on what one senior official calls "substantive credibility." In this view, you fix the credibility by fixing the policy. The White House believes it is crucial for the president to keep his promise to have the website up and operating by the end of this month. If that happens and the health care implementation is seen to be working, they believe the president's ratings will rebound. In this, they get some support from Andrew Kohut, founding director of the Pew Research Center. He ties Obama's current dip to all the publicity attending the health care rollout. "The emphasis in the trends has to be on how people see current conditions," Kohut says. "There is a fair amount of blowback on health care policy, with people being very critical of the way it has come off in its early days, and that has an impact."

But even pollsters urge caution about reading too much into their ratings on "honest and trustworthy." They keep going back to Jimmy Carter, who earlier this year was ranked by Reader's Digest as one of the "most trusted people in America." Carter "had high personal ratings and abysmal performance ratings," reminds Kohut. But he was crushed in his bid for a second term at the same time the public viewed him as honest and trustworthy.

And Newport points to Clinton for the other side of the equation. "Clinton was below 50 percent on this his wholeadministration. He was always perceived as not all that honest. It wasn't that he lost it; he never had it — even though he was elected twice and ended up with very high approval ratings," Newport says. "The public just accepted as part of the Clinton persona that he wasn't all that honest."

The lessons of Carter and Clinton are why there should be great caution today in drawing hasty conclusions on this president's credibility. What influenced voters the most in those two presidencies was not personal honesty. It mattered less that Carter was seen as honest than that the country saw hostages, gas lines, and inflation. It mattered less that Clinton was seen as a rogue than that the country saw balanced budgets, prosperity, and peace.

The fact is, nobody really knows how to measure a president's credibility. And nobody really knows how much it matters to the American public or how much it affects a president's ability to govern. That won't stop today's critics, of course. They will continue to draw sweeping conclusions. They just won't always be credible.