By David M. Shribman, Globe Staff, 2/20/2000

Now the bitter struggle for the Republican presidential nomination - freed from ground wars in small, idiosyncratic states like Iowa, New Hampshire, and South Carolina - enters an entirely new phase.

As the campaign this week moves into the major population centers, it is being fought by two candidates whose political identities have been transformed by the first phase, which ended when the ballots were counted in South Carolina last night.

But, more important, the struggle between Governor George W. Bush of Texas and Senator John S. McCain of Arizona is for the nomination of a party that itself has been transformed - utterly, deeply, perhaps irrevocably - by the events of the past month.

Bush, whose victory yesterday came when he was closer to elimination than his advisers dared admit publicly, is a far different candidate now that he has had to fight for the nomination. McCain, whose loss came at the end of an 18-day period in which he dominated American political discourse, has been transformed into a reformist icon in dire need of a win.

These two men - an easy, mild charmer who has become a pugilist and an intuitive, fiery conservative who has become a populist beloved by moderates and fascinating to liberals - now go forward in their effort to lead a party that is struggling to come to grips with the uncomfortable possibility that its nominee could steer it into the uncharted waters of reform.

And - almost lost in the wild swings from Iowa to New Hampshire and then to South Carolina - a nation that viewed politics as a hopeless bore may be awakening from its decade-long slumber. A poll released last week by the Pew Research Center showed a dramatic increase in public interest in the past month, with the steepest gain coming from Republicans.

The weekend belongs to Bush, of course, for halting the Arizonan, whose dramatic 19-point victory in New Hampshire made him a brilliant, streaking comet in the winter sky and transformed McCain into that most rare of American political phenomena: a proper name that stands for a movement. Bush's victory, which of course could be rendered an ephemeral detour if he loses Michigan and Arizona only two days from now, is nonetheless a triumph of the party establishment that rallied to his cause and that, for a half-century, has ruled Republicans in South Carolina.

But Bush, no less than the campaign and the party, has been affected by McCain and his intoxicating idiom of reform.

Now the Texas governor, who began by talking about ''compassionate conservatism,'' then by emphasizing ''prosperity with a purpose,'' now speaks of being a ''reformer with results.'' He was saved yesterday by his blue-chip allies, but he knows that the support of elected officials and a relaxed sense of destiny may have been enough to prevail in earlier GOP campaigns but is neither necessary nor sufficient to win in this one.

Even if McCain does not recover this week or on March 7, when California, New York, Ohio, the New England states, and others vote, he has left his mark on the campaign, party, and nation. He remains the only political figure in a generation, with the possible exception of the Rev. Jesse L. Jackson, who has inspired outsiders, lured the disillusioned to registration desks and polling places, and changed the debate. By asking uncomfortable questions he jarred a comfortable candidate, a comfortable party, and a comfortable country.

Yesterday's South Carolina renaissance for Bush stands as a reminder of the traditional role that the New Hampshire contest plays: scaring the life out of a front-runner who often recovers. The South Carolina primary also underlines the importance of factors that are absent in the Granite State but that are crucial in later contests: ethnic issues, class divides, social conservatives and minority voters.

Now the campaign's outcome is more uncertain than it has been at any time since the first soundings began, two summers ago, in a political atmosphere that is completely unrecognizable now.

And a curious logic has set in. The small states have explored the nature and character of the candidates. But their tentative reactions to them - an embrace of Bush in Iowa, a wild fling with McCain in New Hampshire, a reconciliation with Bush in South Carolina - have served only to defer to the bigger states, where the population more nearly reflects the country at large.