NASHUA, N.H., FEB. 9 -- Stunned by his third-place finish in the Iowa caucuses, Vice President Bush today quickly retooled his basic stump speech, wrapped himself in the mantle of President Reagan and borrowed the slogan "I'm one of you" from the Iowa victor, Sen. Robert J. Dole (R-Kan.).

The Iowa results appeared to throw the Republican contest into an unpredictable high-stakes battle between the newly triumphant Senate Minority Leader Dole and the wounded vice president, whose large lead in the polls already may have begun eroding before the flood of bad news from Iowa.

The strength of both Rep. Jack Kemp (R-N.Y.), whose intensive effort here was beginning to pay off the past 10 days, and former television evangelist Pat Robertson, could further complicate the outcome.

Most party activists maintain that Robertson is not the force here that he was in Iowa because the state does not have a fundamentalist Christian tradition, as Iowa does, and because Robertson has extremely high negative reactions here. Many polls show up to 60 percent of voters here have an unfavorable impression of Robertson.

On the day after Iowa, where loyalty to Reagan pays few political dividends, Dole and Bush portrayed themselves as loyalists to the president, who remains very popular in this small, conservative state.

In a somewhat hard-line speech to the state legislators and in other campaign stops, Dole repeatedly cited his support for Reagan's foreign policy initiatives. He distributed a letter from Reagan dated Feb. 5 thanking him for his leadership in fighting for aid for the Nicaraguan contra rebels. "I am proud of this letter which I received yesterday from President Reagan," Dole said, ". . . I have carried the ball for President Reagan for the past seven years."

For Bush, the defeat produced a marked shift in the tone and message of his campaign. Trying to emulate the Kansas senator's evocative appeal to Iowa voters, Bush told high school students in Hopkinton:

"I was born in Massachusetts, grew up in Connecticut, live across the way . . . in Maine, and I understand New Hampshire. A lot of my platform and program is a New Hampshire platform and program."

"If it hadn't been for {Sen.} Joseph Biden {D-Del.}, I'd have a new slogan for my campaign: 'I'm one of you,' " he added, referring to Biden's plagarism of a British politician's speech. Bush added, "The reason I'd want that slogan is because I am."

He then went on to say "I'm one of you" seven more times in the speech. Later, when asked about how he could differentiate himself from Dole by borrowing the senator's slogan, Bush said, "It was supposed to be funny. Maybe nobody got it." After more questions, he said, "I'll drop the slogan." As for his roots in Texas, where he votes, Bush told a reporter: "Texas? I'm one of them, too." And he said, "Can't vote in Massachusetts. Born there. I'm one of them, too."

Slogan aside, Bush's third-place finish behind Dole and Robertson led to a round of fast-paced consultations this morning with shaken top advisers, who insisted the embarrassing third-place in a state Bush won in 1980 was not a mortal blow, but only a serious one. While Bush said he would not change his approach to New Hampshire, the fallout was immediately evident, both in his campaign and here in the state.

Capturing the changed reality a severe Iowa loss can bring, Garret P. Cowehoven of Amherst, a Republican legislator who supports Dole, said after a Dole speech here, "Bush had the state until last night, but now anything can happen . . . . Dole's on a roll but Iowa is not the determining factor here. Bush had a big leg up and has the establishment, including the governor, behind him."

Bush quickly changed his schedule to spend virtually all his time here the next six days, except for the one out-of-state event that could bring him some New Hampshire help: his regular Wednesday luncheon with the president.

Rich Bond, Bush's political director, who was ensconced in Iowa the past four months to try to stave off defeat, said today, "New Hampshire will fix a lot of things for George Bush one way or another . . . . We've always felt {he} has a national base. I don't think that national base will be shaken to its foundations" by the Iowa results.

Bush quickly overhauled his speech to emphasize his service to Reagan. "I'm one of you because in this state there is, on the Republican side, steadfast, loyal support for the president, and I am that, have that," he said. "When the going gets tough, I have not jumped away from the president for my own personal political gain . . . . "

At the same time, the Bush campaign has sent thousands of New Hampshire voters a brochure showing on the cover only a photograph of Reagan with the words, "He's trusted only one man to stand by him -- through thick and thin." Inside is a photo of Bush standing next to Reagan.

At the recommendation of his advisers, Bush also sought to take credit today for Soviet leader Mikhail Gorbachev's announcement on a possible Soviet pullout from Afghanistan. Bush said he and Reagan had talked to Gorbachev "frankly" about Afghanistan during the December summit.

In other shifts, Bush campaign manager Lee Atwater now plans to spend the next seven days in the state. Although Atwater has described the South as a "firewall" that will protect Bush from losing the nomination to Dole, others in the vice president's inner circle are not at all confident that Bush's sizable organizational and poll advantages in the South will hold up in March if New Hampshire, too, is lost.

After today's soul-searching, one senior adviser said, "Six days is not enough time to turn this colossus around if we are sliding down that slippery a slope," a reference to erosion in Bush's lead in New Hampshire as reflected in the latest Washington Post-ABC News poll. Polling over the last seven days, before the Iowa caucuses, saw Bush's 19-point lead over Dole decline to less than 10 points, with Dole gaining slightly and Kemp picking up well.

Kemp, who has virtually no campaign except the one here, began the latest in a round of negative ads today, portraying Bush and Dole as being in favor of tax increases, Social Security cuts and increased oil prices. The campaign's hope is that Bush will collapse and Kemp will become the alternative to Dole. Said Roger Stone, a senior Kemp adviser here: "Our goal is to have New Hampshire's voters see Dole as an unacceptable alternative to Bush. As Bush collapses, his moderate voters go to Dole; his conservative voters go to us. A two-man, Dole-Kemp race."

In the Dole camp, advisers were hopeful that they had pierced the aura of invincibility that Bush had sought to create about his nomination. William E. Brock, the campaign chairman, said, "Bush has got to deal with issues," and insisted that Bush's strategy of using the cloak of front-runner status and the vice presidency to avoid issues will not work after Iowa. "He can't do that," Brock said.

Richard Wirthlin, Dole's pollster, said his surveys show Dole way ahead of Bush on such character issues as "strength," "capacity to make a difference," "leadership" and on a commitment to avoid raising taxes, although Dole has been more open to a possible revenue increase. The one area Wirthlin was willing to identify where voters give Bush the edge is on foreign policy, the subject of Dole's speech to the legislature today.

Looking relaxed as he flew in here, Dole claimed to reporters "we don't run negative advertising in New Hampshire," despite the fact that he is running a commercial in which a photo of Bush steadily fades out, suggesting that the vice president has not played a major role in national events.

"It's an official photograph," Dole said with a smile. "It's a good picture."

The negative Dole ads have worried some top Bush advisers, who are considering whether to scrap Bush's television advertising and take a more pointed approach to Dole on the airwaves.Staff writers Thomas B. Edsall, Bill Peterson and James R. Dickenson contributed to this report.