Faced with the interminable period of 21 days without a major primary, Washington's political buffs have been battling withdrawal symptoms by tossing around scenarios under which a Democratic National Convention floor fight over an arcane rule could determine the party's presidential nomination.

The various plots, all based on the hypothesis that President Carter will win a majority of Democratic convention delegates in the primaries while continuing to slump in opinion polls, involve a procedural ploy in which Sen. Edward M. Kennedy would try to snatch away delegates committed to Carter.

At the heart of the schemes is the prospect of a dramatic floor fight over convention rule F(3) (c) , designation that could enter the roster of political trivia right behind 16-C, the rule that sparked Ronald Reagan's futile, last-gasp floor fight against Gerald R. Ford at the 1976 Republican convention.

The Democrat's rule F(3) (c) says that convention delegates must vote for the presidential candidate they were elected to support at least on the first convention ballot. According to Kennedy campaign workers and kibitzers from the press and the polictical industry, if the Kennedy forces successfully challenged that rule from the floor, this would allow delegates from states Carter won to cast their ballots for Kennedy.

This assumes, of course, that enough Carter-pledged delegates would want to vote for Kennedy to give the senator the nomination.

There is no clear evidence at the moment to support that assumption, but it is at the heart of Kennedy's underdog strategy for snaring the nomination.

The Kennedy forces know that even if their candidate wins most of the remaining primaries, Carter is likely to end up with a majority of convention delegates because of the big delegate lead he has already built up.

But Kennedy staffers like Paul Kirk, the campaign's chief strategist, and Patrick Lucey, the deputy campaign manager, are suggesting Carter may be so seriusly hurt by future primary losses, inflation, and foreign problems that he will be widely viewed as an underdog for reelection by the time the convention opens on Aug. 11.

"By then," Lucey told a group of reporters yesterday, "you will have a Republican ticket in place and you could have polls showing that Carter cannot beat that ticket. And in that case, I don't think the Democratic convention is going to nominate a man who clearly cannot win."

That leaves the problem of the Carter-bound delegates-- which is where the floor fight comes in. Lucey pointed out yesterday any convention rule can be changed by a majority vote of the delegates. "There is a Rules Committee, but its decisions are subject to ratification by the convention," he said.

The floor-fight narrative was first proposed by Kennedy himself three weeks ago, when he first said he would take his campaign to the convention even if Carter won a majority of the delegates. "Those of us who've been to these conventions know," he said, "you can do a lot with the rules if the delegates want to come along."