MANCHESTER, N.H. -- The Iowa cataclysm is shaking hard-rock Republicans here, strengthening the conviction of George Bush's critics that he is nothing more than a hollow clone of Ronald Reagan and bolstering the threat of Pat Robertson to mainstream Republican conservatives.

The beneficiary of these deep fault lines running through the New Hampshire party is Sen. Robert Dole. Nightly telephone tracking by the Dole campaign shows that Vice President Bush's early 25-point lead is being cut in half every day. As of Thursday morning, that once-impressive margin was down to 4 points and disappearing.

That should guarantee a Dole victory here, either by absolute numbers or in terms of media and public perceptions in the expectations game. It also guarantees Republican agony over Robertson's claim to be the standard-bearer for true-believing conservatives, evangelical or not, who made him a stunning factor in Iowa. The first casualty may be Rep. Jack Kemp, the most bona fide Reagan legatee in the race, just as he was gathering strong momentum.

The political perils that confront Bush are everywhere evident. So is the paucity of his restorative powers. Rehabilitation comes down to one simple but probably impossible panacea: wrap himself deeper in the Reagan mantle.

The lastest Bush mailer, dropped through thousands of front doors this week, is a fancy 7 x 7 cutout that at first glance shows only a glamorized picture of Ronald Reagan. The Bush presence is discovered by lifting the cutout, where the vice president appears with a legend that ''only one man'' has stood by Reagan's side for the past eight years. That claim could weaken, not strengthen, Bush's uncertain political personality.

Bush did irreparable damage among the pillars of small-town Republicanism here when his Iowa managers attacked Elizabeth Dole in a last-ditch attempt that was aimed at coaxing Bob Dole into a self-defeating counterattack. Watching the vice president Tuesday during an appearance at the Wilkins High School in Amherst, Pete Houston, a Bush backer, was still bothered by the Iowa incident. ''That was a bad mistake,'' he told us. ''It was out of character. That was like Bob Dole, not George Bush.''

Bush has tried hard this week to recover his poise from the humiliating defeat at Robertson's hands in Iowa. But he remains off balance. His good-natured attempts at self-depreciating humor don't come off. His theft of Dole's Iowa line that ''I'm one of you'' rings false. Every Republican here knows he makes the same claim in the South as a voter who has never cast a ballot outside Texas.

His claim to the nomination appears more solitary than ever. It starts and ends with the Reagan link.

But for Bob Dole, arriving here as conquering hero, the transformation this week has been electrifying. Standing at the rostrum under the famed portrait of George Washington in the house chamber at Concord, where he addressed a joint session of the legislature, he followed the advice of campaign boss Bill Brock and his New Hampshire chairman, Sen. Warren Rudman: Be presidential.

At least one tough, anti-Bush television ad has been thrown out. New Dole commercials revert entirely to the ''leadership'' theme, criticizing Bush only by indirection. Dole's tracking shows that the undecided are now down to just over 10 percent. With a digital lead in sight, he is trying hard to conceal the notorious cutting edge of ridicule and attack that is his trademark.

But it is Robertson, not Dole, whom Republican insiders are watching closest, and doing so with more alarm than they publicly admit. At an all-candidate evening in Nashua's Hotel Clarion Wednesday, Robertson's reception by 700 of the faithful was embarrassingly restrained. Not a hand clapped when he said, ''I know about hard-ball politics,'' as though to immerse himself in the Republican mainstream.

No one knows how he will do in Tuesday's primary election, or even how many charismatic Christians there are. But one highly qualified forecast puts him ''in double digits'' and possibly in third place, ahead of Kemp.

Robertson is casting a long shadow here and elsewhere, a shadow weighted with ominous implications for party leaders. One of them, Rep. Vin Weber of Minnesota (where Robertson drew immense crowds early this week), told us while campaigning here for his friend Kemp that the Republicans must recognize Robertson as a "political problem of high magnitude."

Along with elevating Dole to front-runner status, the Republicans who vote here next Tuesday will help define that threat.