MANCHESTER, N.H., FEB. 8 -- Vice President Bush became a vulnerable front-runner tonight.

With a Washington Post-ABC News poll reporting the first clear evidence of eroding support in New Hampshire, Bush's third-place finish and 2-to-1 drubbing in the Iowa caucuses by his main challenger, Sen. Robert J. Dole (Kan.), changed the dynamic in the Republican race.

"It throws everything in the Republican Party in a cocked hat," said New Jersey Gov. Thomas H. Kean (R), a prominent neutral.

The caucuses proved former television evangelist Pat Robertson's ability to attract enough political newcomers to gain a second-place finish and convinced such party leaders as Tennessee GOP Chairman Jim Henry that Robertson "will run a very strong race everywhere he runs."

On the Democratic side, Rep. Richard A. Gephardt (Mo.) scored what party power broker Robert S. Strauss called "a big win," but he did not eliminate his major midwestern rival, Sen. Paul Simon (Ill.), and he left the New Hampshire favorite, Massachusetts Gov. Michael S. Dukakis, in an advantageous position here, facing divided opposition.

Iowa also signaled a virtual bankruptcy sale for the Democrats' two westerners, former Arizona governor Bruce Babbitt and former senator Gary Hart (Colo.). Two other Democrats, Sen. Albert Gore Jr. (Tenn.) and Jesse L. Jackson, will have a chance to show their strength in the March 8 "Super Tuesday" primaries, most of them in the South.

The Post-ABC News poll, taken last week of likely voters in next Tuesday's New Hampshire GOP primary and released tonight, showed Bush leading Dole 36 percent to 26 percent, a significantly smaller margin than in earlier polls.

The trend through the week showed Dole and third-place contender Rep. Jack Kemp (N.Y.) gaining at Bush's expense.

Kemp's backers expressed disappointment tonight as he trailed Robertson in Iowa, which could block his climb.

For the Democrats, the poll showed Dukakis holding a solid-looking, 3-to-1 lead over Gephardt and Simon, who were running ahead of him in Iowa. Dukakis had 43 percent to Simon's 13 percent and Gephardt's 12 percent in the survey.

The four trailing Democrats and three trailing Republicans were in single digits, but Robertson and Babbitt hoped to improve their prospects by edging into the top three in Iowa. Robertson did so; Babbitt did not.

Since the Iowa caucuses became important media events in 1976, swings of 25 percent between the perceived winner and loser there have been measured in New Hampshire polls. Those who finish below the top two in Iowa generally have been unable to hold their strength in New Hampshire and often have suffered severe damage from what is called the "Iowa bump."

Former secretary of state Alexander M. Haig Jr. said the Iowa results showed that "Bush's support is a mile wide and a foot deep. This could be the start of political hemophilia for him." Haig, who pulled out of Iowa and is last in New Hampshire polls, may be a prejudiced source.

But Republican political consultant Eddie Mahe, a neutral, called the Iowa results "a real big hit" to Bush's prospects, saying it is imperative that "he hold onto first in New Hampshire and win it respectably. There aren't a lot of people who are intense, emotional and excited about George Bush. They can't go very long without a victory."

Paul Jacobsen, manager of Dole's campaign here, said in advance that he expected the Iowa results to mean "more than ever before" because of the extraordinary intensity of coverage by Boston and Manchester news media, largely because of Dukakis' candidacy. "Iowa has never had as much visibility as this year," he said.

Ron Kaufman, Bush's New England manager, said the campaign had called its local chairmen throughout the weekend "telling them Iowa doesn't look good; we could come in third" behind Dole and Robertson.

But Kaufman argued that "we have planned from Day One on losing in Iowa and coming back here, and that is what is going to happen." Widespread expectation of a Dole victory, he said, will reduce its psychological impact, because "the bump is a factor of surprise."

Bush has been a double-digit leader in popularity polls nationally and, according to year-end spending reports, had about $11 million available, compared with almost $7 million for Dole.

The Post-ABC News poll, however, indicated that Bush is seriously vulnerable in New Hampshire, where he was trounced by Ronald Reagan in 1980 after winning Iowa. Bush led Dole by 20 percentage points among 150 Republicans and independents who were interviewed last Monday and Tuesday and said they would vote in the primary.

That was the same margin as in a larger poll concluded by The Boston Globe last Wednesday. By Thursday and Friday, however, when the last 232 Republican voters were interviewed by The Post and ABC News, Bush's margin was down to 7 points.

The sample is too small for firm conclusions but strongly suggests movement away from Bush and toward Kemp and Dole. The survey of 501 voters placed Kemp third with 13 percent, former Delaware governor Pierre S. (Pete) du Pont IV with 9 percent, Robertson, 6 percent, and Haig, 3 percent.

Kemp's standing in the poll improved steadily each day, with the impetus largely from self-identified conservatives. Many observers have considered his potential in New Hampshire far greater than that of Robertson, but today his state manager, Paul Young, said Kemp had "to be close to" Robertson in Iowa to hold his position.

Tonight, he maintained that Kemp was still "the conservative alternative" in New Hampshire, saying, "Robertson cannot match this performance in a primary state."

Robertson's local spokesman, Bob Stiles, said canvassing here showed only light support or opposition to any candidate among two-thirds of the Republicans. "Now that he has become the Iowa surprise," he said of Robertson, "a lot of his negatives will disappear."

Many analysts attribute Bush's strength in New Hampshire, as in other places, to his links with President Reagan, but it is not clear whether he will have sufficient appeal on his own, after a loss in Iowa, to hold the Reagan coalition together.

By contrast, Dukakis appeared to be in a somewhat stronger position to withstand the habit of New Hampshire Democrats to stun early favorites, as they did Walter F. Mondale in 1984.

New Hampshire Democratic Chairman Joseph Grandmaison said Dukakis' "support here is very broad and very deep, and it's been enhanced by the skill of his local campaign."

The Globe poll, which showed Dukakis 21 points ahead of Simon and 29 ahead of Gephardt, found that, in the last month, Dukakis' already high ratings for qualities of strong leadership, good judgment and caring for people's needs had gone higher -- above 80 percent for each.

Nonetheless, the apparent third-place finish in Iowa could make Dukakis somewhat vulnerable here, in the view of many observers. The relative closeness of the Democratic race and the unresolved struggle between Gephardt and Simon for the role of prime challenger mitigated the worst of the threat to Dukakis, who is far better financed than either of them.

"If Dukakis were third and not close, we'd certainly have to deal with an expectations problem," Charlie Baker, his New Hampshire manager, acknowledged today. "But we always assumed someone would be coming out of Iowa with momentum to challenge us."

Gary Galanis, Simon's local spokesman, said finishing in the top three in Iowa would provide the boost needed by Simon here. "We have momentum now in New Hampshire, and our polls show us a solid second with 14 to 18 percent."

Mark Longabaugh, Gephardt's manager here, estimated that Dukakis' core strength is probably no greater than one-third of the New Hampshire Democrats, leaving plenty of room for someone else to win.

He expressed confidence that the same populist, antiestablishment message that powered Gephardt's late surge in Iowa would work here in a more prosperous economy. "Even with 3 percent unemployment," he said, "there's a lot of anxiety out there. And Dukakis is the establishment candidate here."

But Dukakis has a big advantage in cash over his badly strapped opponents, who are reportedly coming out of Iowa with empty wallets. Baker vowed that whoever leads the opposition, "we're going to take it to him from Day One" in New Hampshire.

None of the four trailing Democrats in the Post-ABC News poll in New Hampshire gained any lift from Iowa. Hart, winner here in 1984, was at 7 percent; Babbitt and Jackson had 6 percent each, and Gore, who pulled out of Iowa, was at 5 percent. They appear almost foredoomed to trail again next Tuesday.

Hart, who spent the evening here, said, "I'm in the race to stay. I think we're going to do better and better and better the longer this race lasts."

Babbitt's New Hampshire chairman, Mike Muir, taking a more realistic line about the Arizonan's prospects, said, "Obviously, we wanted to have done better in Iowa . . . . We won't make any decisions until after New Hampshire."

The mixed results in Iowa were hailed by Gore's national manager, Fred Martin, as vindicating Gore's decision to fold his campaign in that state.

Staff writers James R. Dickenson and Gwen Ifill, polling director Richard Morin and researcher Colette Rhoney contributed to this report.