AT A GARISH REPUBLICAN FUNDRAISER luncheon a couple of weeks after the primaries, U.S. Senate hopeful Jim Rappaport was hard at work.

While the rest of the Republicans' slate of candidates--from an aloof William Weld to an effusive Joseph Malone--were trying to sneak in a few solitary moments with their filet mignon at the head table, Rappaport was out among the well-wishers, pressing the flesh and accepting discreetly-offered envelopes with a grin.

Rappaport--the GOP's 34-year old challenger to one-term Democratic incumbent John F. Kerry--never managed to touch his steak; a tuxedo-clad waiter silently whisked it away while the candidate was working the crowd. Still politicking, Rappaport managed to miss dessert as well.

In fact, he missed the entire $1000-per-plate meal.

Meals, though, aren't all that James Rappaport--"Rappy" to Republican buddies--is missing. He's also woefully short on credibility: Very few political analysts, and certainly not his opponent, are taking the Concord real estate developer seriously as a politician.

Rappaport must be shaking his head in amazement. Until a few weeks ago, his chances to unseat Kerry looked promising. He had steamrolled over his opponent in the GOP primary, Dan Daly, by scooping close to $2 million out of his personal savings. And entering the general race, Rappaport appeared to be in a virtual deadlock with Kerry, according to several polls.

But instead of soaring past his rival, the Republican challenger has fallen nearly out of the race. Thomas Kiley, a Boston political analyst, says that the candidate "is now almost at the point of unelectability.

How did it happen?

IF EVER ANYONE HAD AN ideal chance to campaign against a Congressional incumbent, it was Rappapport. Kerry, like all his peers up for reelection, had to stay in Washington until Congress and the White House agreed on a budget. During that time, he had been unable to campaign face-to-face against his opponent.

For weeks after the primary, Rappaport has had Massachusetts to himself, like a schoolboy at recess while the rest of the class stayed in detention.

And Kerry's absence was telling. At his Boston campaign headquarters, phones rang endlessly and messages were lost or forgotten. While Rappaport splashed full-page ads onto the Boston Globe and payed for high-tech TV advertising, Kerry's campaign machine seemed to be stalled.

With his opponent safely out of town, Rappaport latched onto a strategy strikingly similar to the one John R. Silber had just employed to grab the Democratic nomination for governor. Instead of sparring with Kerry on the battelfield of national issues, he would equate his opponent with "tax and spend" state liberals like Gov. Michael S. Dukakis.

By capitalizing on voter discontent with state management, Rappaport strategists reasoned, they could ride the same tide of voter anger that had just swept away Democratic incumbents across the state, from Attorney General James Shannon to House Speaker George Keverian.

Of course, voter wrath was directed mainly atstate politicians for throttling the stateeconomy, and not at federal legislators likeKerry. But by campaigning as if he were fightingfor a state senate seat, Rappaport hoped tocapitalize on the same voter anger that seemed towork for Silber.

He started early. On the night of the primary,Rappaport called Kerry "Mike Dukakis's ambassadorto Washington."