"I don't think it's appropriate to challenge the man's credibility in that context," the GOP presidential nominee told reporters on his campaign plane. His advisers were among those who supported the ad's concept early on, only to reverse course at the last minute.

The ad, which had been sent to television stations across the country, featured a 1994 interview with Gore in which he is questioned about calling Oliver North a "pathological liar." Gore is asked whether he and Clinton have always told the truth, and Gore says they have.

But at the 11th hour, advisers to Bush and at the Republican National Committee objected to the spot, arguing that it used an outdated interview and conflicted with the Republican presidential candidate's promise to stay positive, according to several senior Republicans familiar with the ad strategy. These officials spoke on condition of anonymity.

Outside experts said Bush and the GOP were smart to pull the ad, which was to begin running as soon as Thursday. The spot was not only misleading, they said, but was unlikely to be effective.

"It would open Republicans and Bush to the charge that he has broken his pledge to run a different kind of campaign," said Kathleen Hall Jamieson, dean of the Annenberg School for Communication at the University of Pennsylvania.

Campaigns generally don't air personal attacks until the end of a campaign, and only if they're losing, she said. "The perception would be that they're desperate."

The ad was to have been paid for by the Republican National Committee, but as the last-minute decision demonstrated, the Bush camp has a major influence over the RNC's decisions.

The ad featured a television, sitting on a kitchen table, broadcasting the 1994 NBC interview with Gore. In the midst of several questions about North, who lied to Congress about arming the Nicaraguan contras, Gore was asked:

"Can you say that neither you nor President Clinton has told a lie in your political career?"

Gore: "None spring to mind."

Question: "And President Clinton has not uttered a single untruth in the last two years?"

Gore: "Not that I have heard, absolutely not."

The ad did not mention the context of the interview or indicate for viewers what period was covered by "the last two years." The Lewinsky scandal broke in January 1998.

The ad was ready to go, awaiting final approval. But at a meeting Wednesday, officials at Bush's campaign and the RNC objected. They argued that the ad was misleading, took Gore's comments out of context and did not present their best argument that Gore is not credible. Better to use issues, they argued, such as Gore's shifting views on gun control.

Republicans tested the ad in a focus group and the results convinced some GOP officials that independent voters and conservative Democrats would have troubles with the ad.

Strategists recalled that anti-Clinton ads in the 1998 congressional elections that reminded voters about the Lewinsky episode backfired on Republicans.

Adding to the momentum against the anti-Gore ad, an attorney from NBC's standards department phoned the RNC to inform officials that the network does not allow any of its footage to be used by political parties, said Barbara Levin, spokeswoman for NBC News.

In any case, experts said the ad wasn't smart politics.

"I think most people see that there's a difference between Bill Clinton and Al Gore," said Bill Benoit, who studies political ads at the University of Missouri. "I don't think they're worried that Gore's going to have an affair and lie about it."

Republicans may be figuring that out, said Stephen Hess, a political analyst at the Brookings Institution in Washington. The Bush campaign, in sharp contrast to the RNC ad, is airing upbeat spots about education and making tough choices.

"Either out of instinct or out of focus groups," Hess said, "Republicans have figured out that mainstream America doesn't want to hear about Monica Lewinsky any more."