A commercial from my now-distant youth averred: “If you’ve got your health, you’ve got just about everything.” (Note to ad makers: recall and persuasion are not the same. I recall that line vividly but would not be able to tell you the product on whose behalf it was deployed if my life depended on it.)

I would like to propose a corollary to that ad: If you’ve lost your credibility, you’ve lost just about everything. George Bush has lost his, and with it the possibility of regaining his once strong hold on public opinion.

While voters do not see politicians as paragons of virtue, they tend to give individual elected officials the benefit of the doubt on basic matters of trustworthiness. Even before Sept. 11 transformed his image, back when he was just a president who had the lost the popular vote to Al Gore, two-thirds of Americans said Bush was honest and trustworthy.

However, what transpired since is a catalog of dishonesty seemingly unparalleled since Watergate. There really are too many to review here. A few examples will have to stand for the many.

The president told us there were no doubts Iraq had weapons of mass destruction. Wrong. Now the president may or may not have known he was misleading the country on this question (though a majority believes he did so knowingly), but he was knowingly dishonest about the degree of certainty around the question of WMD.

President Bush knew the stories he told about Iraq’s search for nuclear material in Niger and aluminum tubes for missiles were not true, which brings up another falsehood. The president and his minions often swear they pay no attention to polls, yet why did the Niger-nuclear weapons story become important enough to lie about? Because of the polling. Polls told the president that the strongest case for war rested on the claim that Iraq had not just any old WMD but nuclear weapons specifically. Preserving the fiction of a nuclear Iraq was apparently worth lying about from the president’s point of view.

Witness next the related spectacle of President Bush earnestly proclaiming his desire to get to the bottom of leaks of classified information, even though, we now learn, in the inimitable phrase of Rep. Jane Harman (D-Calif.), that the president himself was the leaker in chief. Sixty-two percent believe Bush acted unethically in this matter.

This president’s dishonesty goes far beyond Iraq, of course. No one could imagine, he told the nation, that New Orleans’s levees would be breached. Yet there he was being told on tape just days before that such a disaster could well ensue.

Dancing on the head of a pin, his defenders noted Bush had been told that the levees could be “topped,” not “breached.” But, of course, topping leads to breaching. Indeed, in the days immediately after the disaster we were told the levees had been breached because they had been topped. Semantic gamesmanship would not salvage either New Orleans or the president’s reputation for integrity.

As a result of these and other breaches of the public trust, the number who say the president is honest has shrunk by over 20 points since the beginning of his term and by even more since his post-Sept. 11 honeymoon. A majority of Americans now believe their president is not honest.

Because credibility is so much more easily lost than regained and because it is so central to our evaluations of others, George Bush will likely leave office as the least popular president since that chopper whisked Richard Nixon away from the White House to ignominy.

Mellman is president of The Mellman Group and has worked for Democratic candidates and causes since 1982, including Sen. John Kerry (D-Mass.) in 2004.