Texas Gov. George W. Bush, struggling to contain the first crisis of his front-running presidential campaign, said yesterday that he has not used illegal drugs at any time during the past 25 years.

Bush's response marked a sudden and unplanned shift in strategy forced by specific and unexpected questions over a 24-hour period. He was asked whether he could meet the same standards regarding past drug use that are applied to current senior government officials and that would be required of appointees to a Bush administration.

Caught between his long-stated refusal to answer questions about drug use and the appearance of embracing a double standard, Bush quickly shifted ground, not once but twice in two days.

"Not only could I pass the background check and the standards applied to today's White House, but I could have passed the background check and the standards applied on the most stringent conditions when my dad was president of the United States, a 15-year period," Bush said while campaigning in Roanoke.

He also made clear he could have passed the test at the time his father was inaugurated in 1989.

The questions about government background checks caught Bush by surprise and appeared to rattle the campaign. Whether Bush's answers were sufficient to dampen a controversy that has swirled around his candidacy from its start was not clear by the end of the day.

Some of Bush's opponents, Republicans and Democrats, have quietly asserted for months that they believe the front-runner's biggest vulnerability lies in what he did in the past, though Bush has privately reassured some top supporters that his "youthful mistakes" did not involve hard drugs and would not disqualify him to be president, according to several sources.

While effectively ruling out drug use since 1974, when he was 28, Bush refused to answer other questions about whether he had used illegal drugs before then, questions that government employees seeking security clearances are required to answer. He said voters could make their decisions about his fitness for office. "I am going to tell people I made mistakes and that I have learned from my mistakes," he said. "And if they like it, I hope they give me a chance. And if they don't like it, they can go find somebody else to vote for."

After an afternoon news conference in Columbus, Ohio, that was again dominated by the issue, Bush said he would have nothing further to say on the subject. "I believe it is important to put a stake in the ground and to say enough is enough when it comes to trying to dig up people's backgrounds in politics," Bush said. He added: "What you just heard from me is my answer for the course of the campaign."

Bush campaign officials hoped that the furor over the drug issue the past two days would begin to cool the controversy and argued that after one misstep on Wednesday they had succeeded in reinforcing the impression that, if Bush ever used drugs, it was when he was a relatively young man.

"We're actually better off now than we were 48 hours ago because we've now pushed the story back a quarter of a century and demonstrated that Governor Bush will not ask anyone who serves in his administration to meet a standard that he is not able to meet himself," said Ralph Reed, a GOP strategist and Bush adviser.

But other Republicans, even those friendly toward the governor's candidacy, doubted that yesterday's limited response would quell the controversy. "I think it's going to lead to more and more questions," one strategist said. "Now the question is what happened between ages 18 and 28. I don't think those questions will stop until there is an answer."

One of Bush's GOP rivals, Utah Sen. Orrin G. Hatch, urged the Texas governor to put the cocaine issue to rest once and for all. "I think he is going to be beaten to death with that question until he finally has to answer it," Hatch said. "If he didn't use cocaine, then say he didn't. If he did use it, then explain why he did, and that it was a terrible part of his life, and show how he has overcome it."

Other Bush rivals were more circumspect, but several of their advisers said they believed the issue would continue to dog Bush and that the party should not rush to make Bush the nominee until all questions about his past have been answered.

"The question Republicans have to ponder is, after the Democrats get done with this issue next year, what does the party do if the headlines read, `Bush Lead Down to a Nose'?" one rival strategist said. "The party needs to be careful."

Bush's allies rallied behind him yesterday. "Governor Bush says he has made mistakes," said Utah Gov. Mike Leavitt through a spokeswoman. "I accept that. He says he has learned from them. I believe that. He says he doesn't want to talk about it. I respect that."

Michigan Gov. John Engler, a senior Bush adviser, said that answering specific questions about drug use in Bush's youth would only feed an appetite to put down other rumors. "This is a game . . . where we use gossip, rumor and innuendo to bring somebody down," he said. "Once you start down that road [of responding], what's next week's rumor and the next?"

Bush advisers believe that, in the wake of the Monica S. Lewinsky scandal, the public has little appetite for press inquiries about the private lives of politicians, particularly in the absence of credible allegations, and they argue that the current flare-up will die out within a week or two.

Bush has been the subject of repeated rumors involving drug use, but there is no evidence to back them up. A number of news organizations, including The Washington Post, have investigated the rumors and found no proof.

"If anybody is skating on thin ice on this issue, it's the media and not the candidate," said Fred Steeper, a Bush pollster. He would not comment, however, on whether he had done any polling on the issue.

Recent opinion polls show that the public believes that questions about drug use by presidential candidates are legitimate, though they do not regard the answers as highly relevant in determining a candidate's fitness for office.

In a Fox News/Opinion Dynamics poll released last week, 69 percent of the registered voters surveyed said they wanted answers to the question of whether a candidate had used cocaine in the past. But 72 percent said experimentation with cocaine as a youth should be forgiven.

Frank Newport of the Gallup Organization said a poll from last February found that while 53 percent of those questioned wanted to know about past drug use among politicians, they were more interested in learning about such things as whether a politician cheated on his taxes, failed to pay his debts, was an alcoholic or had been accused of sexual harassment.

Said James Carville, who has been at President Clinton's side through many scandals: "In the end, he's going to have to mess up a lot more than he's messing it up now for it to hurt."

The Bush campaign was clearly caught off guard by the sudden intensity of the questions this week, and it blamed his opponents for trying to slow his momentum toward the nomination in the aftermath of his victory in the Iowa straw poll on Saturday.

In the past, Bush has talked openly about alcohol abuse -- he quit drinking in 1986 -- and has volunteered that he has been faithful to his wife. But he has refused to respond to questions about drugs. The candidate was pressed again on Wednesday in Austin and vigorously responded by saying he would not play "the game" of responding to rumors.

But later, a Dallas Morning News reporter asked how he would respond to a standard government questionnaire used in background checks that asks potential employees about drug use during the previous seven years. Bush did not immediately answer the question, in part, aides said, because he did not understand it.

But later he told the newspaper: "As I understand it, the current form asks the question, `Did somebody use drugs within the last seven years?' and I will be glad to answer that question and the answer is `No.' "

Bush and his team hoped that would put the issue to rest, but they were alarmed when they woke up yesterday to see that news reports focused on Bush's saying he had not used drugs during the past seven years -- not his commitment to enforce government background checks and to meet the same standard. Bush's denial of drug use over the past seven years appeared to leave open the possibility that he had used drugs while his father was in the White House.

That led to a revised strategy and to the news conference in Roanoke, where Bush said he could pass the test used by his father's administration, which initially disqualified senior-level applicants for drug use within 15 years of joining the administration -- and could have passed it at the time his father was sworn in.

Asked why he was suddenly willing to answer specific questions, Bush responded, "It was a relevant question about how I would handle background checks if I were president."

Gray said the Bush administration used a two-step process to determine fitness on the drug issue. All applicants were required to reveal any drug use at any time in their life. If the drug use was in the distant past, it was not considered disqualifying. But Gray said that during the course of the administration, the standard changed from 15 years to 10 years. "We changed the standard because it was onerous to get people for the administration," he said.

Bush's answer yesterday fell short of the standard required of senior government officials both in the Bush administration and in the Clinton administration, who must reveal drug use back to age 18.

Polling assistant Claudia Deane and staff researcher Ben White contributed to this report.