An exhausted, ecstatic George Bush rushed from Des Moines and morning-after victory interviews over national television to New Hamphire and a world he had not faced before: huge but skeptical crowds, Ronald Reagan actually on the scene and hard-nosed enemies who take him seriously.

No sooner had Bush's stunning win in the Iowa caucuses been confirmed than Reagan's New Hamphire operatives sent word west: he must campaign heavily here or suffer a second -- perhaps fatal -- defeat. Reagan's previous promise of eight more days in the state was promptly doubled to 16. What's more, his operatives here are ending glacial disdain of Bush, substituting personal attacks.

How well Bush will adjust to this new world was put in doubt by his early performance as Reagan's Iowa conqueror. Obviously tired, he did not maintain his dynamic pace and cooled off voters who had come to be convinced. With Reagan on the scene campaigning, it will take more than that to win here.

Bush's post-Iowa euphoria was buttressed by telephone calls to his campaign offices around the nation from bandwagon-leapers. The most prominent new ally: respected South Carolina party war horse Harry Dent. But to confirm his breakaway from the Republican pack, Bush needs at least a strong second in New Hamphire's Feb. 26 primary.

Former New Hamphire governor Hugh Gregg, Reagan's astute 1976 campaign manager, has painstakingly built a statewide organization for Bush that includes many erstwhile Reaganites and seems superior to Reagan's present apparatus. But Organization counts for less in New Hamphire's primary than in Iowa's caucuses and Gregg has fretted about Bush's anemic voter identification.

Iowa's triumph ended that problem, as shown by the crowd generated here the next day by the man who last year gave hour-long speeches to a dozen voters. As one of the mild winter's biggest snowstorms began, over 200 voters attended a Walpole reception. Two hours later, 1,200 (many braving treacherously icy roads) jammed into Keene State College's student union. Every seat was taken the next morning at a Chamber of Commerce breakfast in Nashua.

The exhausted candidate was admittedly not his best at Keene State. But even after a good night's sleep at Gregg's home in Nashua, he seemed distracted by his new eminence. In Iowa, he delivered Reaganite pronouncements on economics and foreign policy more forcefully than the current Reagan; after his triumphant return to New Hampshire, he reverted to the fence-straddling that has plagued moderate Republicans.

The breakfasting businessmen in Nashua came to be impressed and left disappointed by Bush's repetition, imprecision and refusal to take a hard position on issues ranging from child daycare to nuclear power; the room cooled when Bush declared he didn't know enough to have an opinion on the Seabrook, N.H., nuclear power plant. A consulting engineer from nearby Amherst told us before hearing the speech that he had reduced the field to Reagan and Bush, but planned "to cement my choice for Bush today" because of "better experience and more smarts." After the speech, his Yankee verdict: "Too much waffling. It's back to Reagan." t

Bush cannot stand many such performances, considering what is being planned for him inside a shabby frame house in a run-down Manchester residential neighborhood. That is Reagan's state campaign nerve center, run by Jerry Carmen, a veteran New Hampshire political practitioner famed for tough tactics.

Carmen, who long ago made clear he would not abide Reagan proconsuls sent by national headquarters (as was done in Iowa), insisted on Reagan's doubled exposure here, beginning Jan. 27. Furthermore, there will be no "imperial candidacy" on the Iowa model; he has scheduled questions-and-answers at every stop, plus long hand-shaking sessions on the new hampshire model (though Carmen opposes Reagan's joining a multi-candidate debate).

Reagan will presumably stick to his 11th commandment against speaking ill of another Republican, but that does not bind Jerry Carmen. The campaign's thrust will question whether George Bush is fit to be president or is the finished product of expert public relations. Carmen contends that Bush, having broken out of the pack, deserves close scrutiny.

The focus of this assault is Bob Goodman, a Bush television consultant who is credited by Carmen with having transformed Bush's personality. On the day Bush arrived from Iowa, he and Goodman were bludgeoned on page one of the Manchester Union-Leader by publisher William Loeb (an ally of Carmen) and by Leob's new feature writer, former governor Wesley Powell (renowned for prolific shedding of Republican blood over a generation).

Gregg welcomes the assault of his arch-enemy Loeb, but that conflicts with memories of what the Union-Leader did to Nelson Rockefeller, Edmund Muskie and, most recently, Philip Crane. From the gentlemanly caucuses of Iowa, Bush has entered the bloody primary battleground of New Hampshire, where Reagan's men know his dilluted myth of invincibility cannot withstand another setback.