In a banquet hall in this Bucks County town, where some 80 people had paid $500 each to hear him plug Jim Coyne, the Republican candidate for Congress, former president Gerald R. Ford on Wednesday marked an anniversary few other Americans would have noted.

"It was exactly four years ago today," he said, "that candidate [Jimmy] Carter came up with what he called 'a good reason to beat Jerry Ford.' It was the 'misery index,' a combination of inflation and unemployment. It was 15.8 percent then, and he talked endlessly about it. It probably had some effect.

"Well, just as a matter fo curiosity," Ford said, as the crowd chuckled appreciatively, "with no ulterior motive I looked up what it is now . . . and it came to 21 percent. If 15 percent was a good reason to beat Jerry Ford, then 21 percent is a darn good reason for turning Jimmy Carter out of the White House."

That pitch, with minor variations, is at the core of Ford's campaigning, as assault by a former president on his successor that is unmatched in intensity in American political history.

Ford is campaigning 53 of the 64 days between Labor Day and Election Day, covering 30 states and 60,000 miles. "I will have 10 joint appearances with [Ronald] Reagan and [George] Bush," he said in an interview, "and help in 14 Senate campaigns, 29 House races, four governors races and I don't know how many party affairs."

The strain of the schedule shows on the 67-year-old Ford's tanned face. The Willow Grove stop was halfway through a brutal five-day cross-country swing that started with an overnight flight from San Diego to Pittsburgh and will include stops in West Virginia, Pennsylvania, New Jersey, Minnesota, Idaho and California.

Ford turns his own exertions into a parable that seems to make his customary audience of Republican donors and workers reach deeper into their own wallets and reserves of energy.

"Retirement is not at all bad," he tells them. "I fall down less on the ski slopes and I hit fewer spectators on the golf course now. After 13 congressional campaigns and one presidential race, I could have sat this one out. But when Betty and I talked about what has happened to our country since Jimmy Carter became president, we decided we were going to do all in our power to elect Ronald Reagan."

As that sentence suggests, Ford's motivation stems mainly from his disrespect for the man who beat him. Carter's name occurs 10 times as often as Reagan's in Ford's speeches. The indictment is sweeping.

"I have not found one world leader who has confidence in the leadership of the United States under the Carter administration," Ford says. "Mr. Carter kept only one of his campaign promises -- a significant reduction in our plans for military strength, both strategic and conventional. . . .And as for the domestic economy. . . ."

After his defeat in 1976, Ford seemed to swallow his personal disappointment. His relationship with his successor was correct, if far from cordial. Ford stopped by the White House on each of his visits to Washington, and when Carter asked for help on the ratification of the Panama Canal treaties, Ford gave it.

But privately, his political cronies say, his eagerness to even the score with Carter was evident even then. It surfaced last winter when Ford flirted briefly with the prospect of a late-starting bid for renomination, and again during the Republican convention in July, when he publicly contemplated signing up as the vice presidential candidate with the man he had beaten for the 1976 GOP nomination.

But never has Ford let his personal and political distaste for Carter hang out to the extent he has done in this campaign. In his speeches, he refers to the "conscious deceptions" of Carter's 1976 race and the "untruths" he says Carter uttered.

In an interview, the normally easy-going and tolerant former president excalated -- rather than softened -- his attack. "He [Carter] misled and deceived the American people in the 1976 campaign," Ford said. "And now the chickens are coming home to roost . . . He's going to be held to account."

Ford said he warned Reagan to expect personal attacks from Carter, and was "not surprised" that Carter has implied the Republican nominee might be a warmonger or a racist. "These demagogic attacks," Ford said, "are very indicative of the mean traits that are veneered over by that smiling exterior."

In the interview, Ford appeared less eager to discuss Reagan's campaigning except to say that Reagan has "tightened up his own performances" after a "very bad" start, and has improved his organization by giving "more meaningful responsibilities" to Stuart Spencer and James A. Baker III, leaders of the 1976 Ford campaign who were added to the Reagan staff this summer.

Ford has remained critical of parts of Reagan's tax plan and of the Republican platform sections on abortion and the Equal Rights Amendment.

He has told his close associates that he was "terribly disturbed" at the trend in the Republican Party reflected by the primary election defeats of such moderates as Sen. Jacob K. Javits of New York and Rep. John Buchanan of Alabama.

But those concerns take second place to Ford's personal crusade to drive Carter from the White House.

"I have no ambitions," he told the audience here. "I don't want a job. But our country is at a crossroads, and I sincerely believe that if Jimmy Carter is reelected he would so weaken this country at home and abroad that it would be in considerable peril."