NASHUA, N.H., FEB. 15 -- Vice President George Bush, one-time Yale baseball team captain, readily accepted the challenge from a reporter for snowball target practice in the midst of a chilling New Hampshire blizzard.

Smack! The reporter, David Beckwith of Time magazine, hit the street sign with a bull's-eye on the first throw.

Plunk. The vice president missed. And missed. And missed again.

It was a symbolic moment for Bush that caught the flavor of an agonizing week that will be remembered as the most critical in his two decades in politics.

In the seven days between his third-place showing in the Iowa caucuses and Tuesday's New Hampshire primary, Bush has been struggling to prevent his once-dominant lead here from turning into another defeat, which even his most optimistic supporters acknowledged would cause his national strength in organization and polls to turn to dust.

It was a roller-coaster week for the candidate and his forces, from Tuesday's stoic hopes that Iowa would not make the bottom drop out of the campaign, to Saturday's despair that the contest here was probably lost, to today's confidence that a win was, once again, possible.

The day of the snowballs -- Friday -- was one of the low points in a week of extraordinary crisis and conflict for the vice president. Cautious by nature, Bush was pressed to take risks to save his campaign. He scrambled to find a sharper, new message; to shuck the trappings of incumbency; to persuade himself that he should attack. His campaign organization, meanwhile, was thrown into chaos by the Iowa defeat, and made a series of blunders that complicated Bush's last-minute efforts to bounce back.

Campaign sources said planning is under way for a major overhaul of the Bush organization if the vice president is defeated in a state where only a few weeks ago his top strategists said he was invincible. His senior staff here is reviewing the campaign budget and potential personnel cutbacks. Today, polls showed Bush in a dead heat with Sen. Robert J. Dole (R-Kan.) and his strategists predicted that if the two were tied in the polls when Tuesday dawned, Bush could edge out a victory based on the strength of the large Bush organization here

headed by Gov. John H. Sununu (R).

The sudden deflation of the Bush candidacy began at the end of the Iowa campaign, according to campaign sources and close associates of the vice president. There had been many long-term factors, including the Iran-contra affair, and polling showed even before Iowa that Bush's lead here was beginning to evaporate. But sources said everything seemed to come unglued when George Wittgraf, Bush's Iowa chairman, unleashed a harsh written attack on Dole, accusing the senator of mean-spiritedness, cronyism and sinking the 1976 Republican ticket.

Although the exact origins of the Wittgraf statement remain unclear, Bush and his wife, Barbara, were said to be privately furious. But publicly, the vice president seemed paralyzed over the episode. Out of loyalty to Wittgraf, who had worked for him for nine years, Bush refused to denounce the statement. He also refused to endorse it. He even pretended for a day that he had not read it.

Internally, his strategists saw the final, precious days of Iowa campaigning slip away as the episode dominated the news. It also had another effect on Bush: it made him even more reluctant to accept hardball tactics when he was fighting for his life in New Hampshire.

On the night of the Iowa defeat, Bush flew here, exhausted. He was met by former senator Nicholas Brady, (R-N.J.) and Bush told his staff, "Don't let anybody get down." The next morning, Bush told his chief of staff Craig Fuller to "fix the schedule," and scrap a trip to New Orleans.

But the first day in New Hampshire after the Iowa defeat was a disaster, according to sources. Bush started the day by mimicking Dole's successful Iowa slogan, telling audiences, "I'm one of you." He used it eight times in a morning speech at the suggestion of his old friend Brady. But a few hours later he dismissed it as an attempt at humor and told reporters he would drop it. Some of the advisers did not think it was funny -- in terms of sharpening Bush's message and offering voters a reason to pick Bush over Dole, they felt they had forfeited the first 24 hours.

As Bush flew back to New Hampshire Wednesday after lunch with President Reagan in Washington, he was accompanied by Peggy Noonan, a former White House speechwriter known for her evocative work, including Reagan's memorable address after the Challenger disaster. Noonan listened on the plane to tapes of Bush's stump speeches. She consulted with pollster Robert Teeter and went to work that night on sharpening Bush's message.

Meanwhile, Bush gave instructions to his Secret Service protective detail that he wanted more flexibility to campaign closer to voters, a grass roots approach long favored by Sununu and a variety of other advisers. A small advance team left Nashua's Clarion Hotel to scour the countryside looking for picturesque campaign stops.

Bush had rarely tried to break out of his traveling cocoon, where everything was scripted down to the minute and everything reflected the trappings of his office. Some advisers had been arguing for months that Bush had to choose: Either be vice president, above the fray, or be a candidate and in the fray.

By Thursday, the choice Bush had made was clear. The campaign was learning how to be informal. The advance team had picked out a working class trailer park for his first stop, but when the lead car arrived there Thursday morning, just minutes ahead of the vice president, and no voters were in sight, Bush was directed on to the next site, a lumberyard.

That afternoon Bush delivered the first direct attack on Dole, which Noonan had written. For the first time, he took on Dole's proposed spending freeze, saying it was a "cop-out." This approach was recommended by economist Mi chael Boskin of Stanford University, who has privately been advising Bush.

But aides made a critical mistake: the new speech was late in the day and local and national news media were not alerted. For the most part, the television news Thursday night showed Bush at the lumberyard and at a truck stop, but missed his new speech.

Later that Thursday, Bush taped a half-hour television commercial in the Hollis town hall, where he held an "Ask George Bush" forum, a staged event where members of the audience ask prearranged questions on familiar subjects such as taxes and education. At the end Bush was asked his response to criticism that he is a member of the "old eastern establishment" wing of the party.

Instead of becoming defensive, as he has before, Bush said, "Some of them are going after me because my mom and dad could look after me when we were born. I had nothing to do with that. When I ran for office in Texas they said this guy's from New England. I said, wait a minute, I couldn't help that, I wanted to be near my mother at the time."

The "Ask George Bush" shows had been successful for Bush in 1980 and campaign advisers believed that if enough voters saw Bush in that setting, it could help turn the tide. To promote viewership, some 300 Bush volunteers fanned out to distribute some 80,000 leaflets in areas of the state with high undecided vote asking people to watch Bush on television Saturday night. The campaign bought air time in such a way that most New Hampshire residents could not avoid the show if they turned on their televisions.

Thursday night, Bush dined with Noonan and Teeter at a restaurant in Nashua. While his broadsides against Dole were growing more harsh, Bush said he was willing to try a more personal appeal as well. "They say I was born comfortable," he said Friday morning in a breakfast speech written by Noonan. "And they're right. Not fabulously wealthy, but even they have their trials. John Kennedy once said to a friend, 'You don't know how hard it was growing up in that house.' His parents made demands. Mine did too."

The Friday snowstorm was a harbinger of trouble. It put a blanket on all campaigning, except for the withdrawal announcement by former secretary of state Alexander M. Haig Jr., who called Dole "head and shoulders" over Bush in leadership ability. Socked in by snow, Bush could only go for a walk. When he returned, drinking a beer and chatting with reporters, he watched with chagrin as the Haig line dominated the television news reports Friday night, as they had all day and even into Saturday.

At the same time, media consultant Roger Ailes had brought to Nashua a new "negative" campaign commercial. While Dole had aired a spot showing Bush's picture fading away, Ailes produced a nearly identical ad in which Dole faded. It was rejected immediately, in part because Bush had already borrowed Dole's slogan earlier in the week and his advisers did not want to be accused of mimicking the senator's ads as well. Ailes then came back with a spot claiming Dole had waffled on the new arms control treaty and an oil import fee.

On Saturday morning, as polls showed Bush's situation deteriorating, the advisers urged Bush to approve the new ad, but the vice president was reluctant. In Iowa he had criticized Dole's negative advertising, and the Wittgraf episode backfired. What if this one did too, he asked.

But Bush approved the spot, and Saturday afternoon he aimed another attack on Dole, this time criticizing his record as Senate minority leader. For months, Bush had hardly mentioned his chief rival. Now he was racing against the clock. He was assailing Dole from every direction.

"This is an entirely different campaign today," compared to 10 days ago, said a long-time Bush adviser, noting that pollster Teeter had largely taken charge of strategy. "Somebody else is running it, its candidate is different, its message is different."

Today, on the last day of the campaign, Bush took a final charge at Dole on the budget issue. His final media gambit was to fly in former senator Barry Goldwater, the 1964 Republican presidential nominee, for a last-minute endorsement and a five-minute television commercial. Bush said Goldwater's appearance in New Hampshire was "exactly the medicine that the doctor ordered at the end of a long week or two since that Iowa situation."

And in the shadows of the candidate, the organization that is supposed to deliver began completing its work. "There is not a Republican household in this state we will not have touched," said New England Bush coordinator Ron Kaufman, looking back at what he described as "tens of thousands of phone calls, 100,000 or more households hit with literature, thousands of contacts at the door. . . ."

On Tuesday, Bush will find out if it worked.