House Democratic leaders have sped up their timetable for gun decontrol legislation in an effort to head off a discharge petition that is now 21 votes short of forcing the issue to the floor.

The petition would allow the House to vote on the Senate-passed McClure-Volkmer bill without amendments. The move by House Democrats is aimed at producing an alternative to McClure-Volkmer, which the National Rifle Association has been advocating for seven years and which the Reagan administration has endorsed.

The bill would allow interstate handgun sales and generally weaken the 1968 Gun Control Act, which was passed after the assassinations of Martin Luther King Jr. and Robert F. Kennedy.

Rep. William J. Hughes (D-N.J.), chairman of the House Judiciary subcommittee on crime, said a week ago that his panel would produce a gun measure by May 1. Late last week, however, Hughes announced that he would mark up a bill no later than March 20.

Judiciary Committee Chairman Peter W. Rodino Jr. (D-N.J.), meanwhile, took the unusual step of announcing in a "Dear Colleague" letter that he would "schedule prompt full committee action" on the bill "as soon as it is reported" by Hughes' panel.

Supporters of McClure-Volkmer, which Rodino declared "dead on arrival" last summer when the Senate passed it 79 to 15, say House leaders appear desperate to block the discharge petition before it gains the necessary 218 signatures, those of a majority of House members. These supporters have charged that Rodino planned to kill McClure-Volkmer by keeping it bottled up in the Judiciary Committee.

"It's a pretty obvious political tactic by Hughes," said NRA spokesman Andrew Kendzie. "In seven years he hasn't done a damn thing about the bill. Now Hughes, Rodino and House Speaker Thomas P. O'Neill are looking at those numbers, and it's scaring the pants off them."

Hughes, a former prosecutor, has cosponsored a bill with Rodino that would impose a 15-day waiting period on handgun sales to allow a police check on the background of buyers.

The measure also contains some easing of record-keeping requirements, as does McClure-Volkmer. Hughes said he will produce a bill that contains elements of several bills before his subcommittee.

McClure-Volkmer is opposed by most of the nation's law-enforcement groups, and Attorney General Edwin Meese III said last week that key aspects of it "could be improved." Treasury Department officials said at a crime subcommittee hearing that the bill is flawed.

These reservations are important because if House opponents succeed in amending McClure-Volkmer, it would have to go to a House-Senate conference where, its backers fear, it would die. The discharge petition could avoid this by allowing passage without amendments. The administration, in suggesting changes, has therefore angered some of the bill's strongest supporters.

Rep. Jack Kemp (R-N.Y.), an original cosponsor, said in letters to Meese and Treasury Secretary James A. Baker III that he is "very concerned" that the administration seems to be "backing down" on the legislation. Kemp said administration officials were making "statements that contradict the president's longstanding support for this measure and endanger the chances for enacting McClure-Volkmer."

Some political observers said Kemp could use the issue against Vice President Bush, a Baker ally, if the two compete for the 1988 Republican presidential nomination