Three days after his Iowa upset over Ronald Reagan, George Bush said, "the euphoria kind of ebbs and flows."

Nonetheless, the hero-of-the-moment in the Republican presidential race told press conferences here and in Bangor today that "if I beat Reagan in New Hampshire, has candidancy will unravel and unravel fast" and "I don't think I could be stopped any way."

Whatever happens in the next big contest in New Hampshire on Feb. 26, Bush said, "It will be a two-man race by the time we get to Illinois [on March 18] and I will be one of the two."

That argument assumes either that Reagan's collapse will be precipitous or that Sen. Howard H. Baker Jr. and John B. Connally will not improve on their distant third and fourth-place finishes in Iowa.

But there are others in the Republican Party who think that Bush faces a long haul to the nominating -- and that the impact of his Iowa victory may have been overstated in the first reaction to his accomplishment.

That analysis would seem to ignore the solid organizational effort that went into Iowa and the increasingly self-confident performance or Bush as candidate.

But Douglas Bailey, the political consultant who is masterminding the Baker campaign, said yesterday that even if Bush follows up his Iowa win with a victory in New Hamphire "it would be too early to conclude that he'd have the thing sewed up. . . Everyone knows that Reagan's strength is in the late primaries."

That judgement is based on both geography and history. Of the 30 states that choose their delegates after April 1, 17 are in the South and West, where Reagan has had the base of his strength. It was in those states -- from North Carolina to Texas to California -- that Reagan closed the gap on President Ford in 1976. Ford won the first primaries in 1976, but came within an inch of being knocked over by Reagan's late surge.

This year, as in 1976, Reagan has the advantage of having his home state of California conduct the only winner-take-all Republican primary. Its 168 delegates consitute one-sixth of the number needed for nomination.

But Bush now has the advantage of the Iowa victory, which is producing a fresh inflow of contributions and volunteers.

The Baker forces, on the other hand, are still "fighting our way back into the battle," as Bailey conceded. Financially and organizationally, he said, "we're trying to climb the ladder one rung at a time. Ours is not a 50-state campaign."

Baker's proclaimed goal is not to win in New Hampshire but to do well enough there that he is competitive with Bush and Reagan by the time of the Illinois primary.

When he announced his candidacy last fall, before he recognized the extent of his organizational problems, the Tennessee senator said he would win his first three victories in the Maine caucuses, scattered from now until the end of March, and the primaries in Arkansas on Feb. 16 and Puerto Rico on Feb. 17.

But Bush upset Baker in the nonbinding presidential straw vote in Maine, and Baker is struggling uphill against Bush and Reagan here now.

Bush's two key leaders in Maine claimed today he would win at least 15 and perhaps all 21 of the Maine delegates.

Baker claims an advantage in Puerto Rico. And Arkansas Republicans backed Baker for vice president at the 1976 convention.

David Keene, Bush's campaign manager, said he expected the Arkansas and Puerto Rico results to be "muddled," and if that is the case, "we'll claim it's another setback for Baker."

It is the Southern tier of primaries on March 8 and 11 that many expect to be a roadblock for Bush, but his manager's view is less than apprehensive, and in some cases, quite optimistic.

In South Carolina's March 8 contest billed by backers of Connally as his best chance to beat Reagan, Keene said Bush entered only because Baker did and want no more than to take "a respectable third place." But privately, Bush campaign officials are hoping to enlist big-name South Carolina GOP support within the next week.

Harry Dent the veteran South Carolina Republican leader who is a likely target for Bush's wooing, said this week that, in his only appearance in the state, Bush drew a standing ovation in Spartanburg and "showed everyone he has not overreached himself in seeking the presidency."

Keene said the Bush campaign was bypassing Georgia's March 11 primary, in effect conceding it to Reagan, who won 72 percent of the vote there in 1976.

But the other two Southern primaries in March 11 will find Bush very much in the running. Initially his decision was to concentrate on Alabama, where he has put together an implausible combination of establishment Republicans and Wallace Democrats -- the latter recruited by Charles Snider. George Wallace's longtime campaign manager.

The big surprise being plotted by the Bush campaign may be in Florida, the most important of the March 11 primaries. After his surprisingly strong third-place finish (behind Reagan and Connally) in the Florida convention straw-vote, Bush moved a team of seven field organizers into the state.

Some of his key strategists believe the Florida GOP is "more Midwestern than Southern" and the Bush campaign is now going at the core of Reagan's strength, from the Cuban community in Miami to the strong GOP enclave in St. Petersburg.

If Bush comes into Illinois on the heels of an upset victory over Reagan in Florida, the predicting of a late spring recovery for the Californian could prove radically wrong.