FOUR YEARS ago, Ronald Reagan won 49 percent of the New Hampshire vote in a two-man race against President Ford. Tuesday, Mr. Reagan won 52 percent of New Hampshire's votes, only this time he did it in a seven-man field. For Mr. Reagan, the victory must have been especially sweet after five weeks of reading premature post-mortems on his presidential ambitions.

What accounts for the trouncing of George Bush? Everyone will have a theory. Apparently, Mr. Bush's fall from favor owed something to both his television "debate" appearances -- the one that was and the one that wasn't. He is said to have lost ground at the Manchester show as well as at the Nashua disaster. In addition, his new-found stardom was fairly fragile, and he didn't do much to reinforce it by dwelling so on process and tactics: Mr. Bush seemed to spend most of his spotlight time telling his audience how he would win the campaign rather than why they should choose him as their president.

New Hampshire primary voters, like their caucus counterpart in Iowa and Maine, turned out in record numbers -- huge numbers. For example, in the 1978 general election when New Hampshire was selecting a governor, senator and both congressmen, 261,000 voters voted. On Tuesday -- in a primary -- 255,000 New Hampshire citizens voted. But when you try to figure out more clearly than that what the message is, you run up against the unusual volatility in the electorate. Wide swings in short periods from one candidate to another seem to be the 1980 norm. Mr. Bush's New Hampshire fall from grace was evidently also aided by the fact that he was for most voters a late connection. Because they had not known George Bush until a few weeks ago, the voters had little history and few feelings to fall back on when Mr. Bush appeared alternately indecisive and insensitive at the Nashua forum. So many left his camp and apparently went directly to Mr. Reagan. But what sent them in his direction -- rather than toward the smorgasbord of others -- has not yet been analyzed.

President Carter -- who, himself, knows something about voters' volatility or fickleness -- won again impressively in New Hampshire. Sen. Kennedy failed for the second time this month to defeat Mr. Carter in the senator's native New England; in contests he had previously said he had to win, Mr. Kennedy, who also took a beating in Minnesota on Tuesday, appears to be, as of now, a candidate with irreparable shortages of votes and hope. Once thought to be the strongest Democratic challenger to the president, he now appears to be an especially weak one, yet another tribute to this election year's capacity for refuting expectations. Mr. Kennedy has had a run of bad luck since his announcement of his candidacy last fall. He has also compounded it with a strident, frenetic, grating campaign tone that, for instance, was evident when he claimed "victory" on Tuesday night in a remarkably silly passage. His campaign, if it is not in fact doomed, needs a real revision to make any sense or to have any chance.