ANAHEIM, CALIF., JUNE 7 -- More than 20,000 members of the National Rifle Association, many expressing bewilderment or anger about the organization's worst political slump, are expected here for a weekend convention that could sharply alter their national campaign to protect the right to own firearms.

In a telling measure of the NRA's legislative woes, the state hosting the annual convention has provided some of its bitterest recent defeats, including a ban on assault weapons and a waiting period for rifle and shotgun purchases that have put more limits on California gun owners and purchasers than ever.

"There is a great deal of frustration among the membership, based on a very tough year," said James J. Baker, federal affairs director for the NRA's lobbying arm, the Institute for Legislative Action.

Membership, slightly less than 3 million, has declined by about 250,000 in the last four years, and some longstanding political friends sided against the organization during a Senate vote on assault weapons two weeks ago.

Some NRA members who have long opposed executive vice president J. Warren Cassidy predicted a membership uprising against what they consider Cassidy's timid and underfinanced efforts to stop new gun-control bills in Congress and state legislatures.

"Under . . . Cassidy, the NRA has suffered a host of unprecedented legislative, financial and program fiascos," said John D. Aquilino, an NRA life member who publishes a newsletter, Insider Gun News.

Gun-control advocates, with ill-disguised pleasure at their adversary's struggles, also forecast problems at the convention.

Luis Tolley, western director of Handgun Control Inc., raised the possibility of "a real bloodbath between moderate and hard-line factions." He said most members at the gathering "are likely to be hard-liners, since they go when they are angry, and this year they are angry."

Several NRA officials and members today rejected the notion of a major split. "I don't see any major change in direction," said Ray L. Burden, a retired nursery wholesaler from Canby, Ore., who serves on the board of directors.

NRA President Joe Foss, a Medal of Honor winner from Arizona who is expected to turn his seat over to First Vice President Richard D. Riley, said "we just need to work harder" to overcome what he said was false information spread by politicians and news organizations about the impact of gun control.

Handgun Control Inc., listing its 1990 legislative successes, noted new waiting-period laws for purchase of rifles in California and Connecticut, an assault-weapons ban in New Jersey, a waiting-period bill headed for approval in Rhode Island and new gun-control bills in Iowa.

Foss blamed much of this on alleged media bias in favor of gun control and on advocates such as Sarah Brady, wife of former White House press secretary James Brady, who was wounded in the 1981 attempt to assassinate President Reagan.

Gun-control advocates, backed by several influential police chiefs and police organizations, have contended that the new laws will keep lethal weapons from criminal gangs and deranged individuals. The killing of five children in a Stockton, Calif., schoolyard early last year by Patrick Purdy, an emotionally disturbed welder, inspired many of the measures against assault weapons.

NRA members say criminals can always find weapons no matter what the law and cite the high homicide rate in the District of Columbia, which has some of the the nation's strictest gun-control laws. The NRA has supported efforts to identify felons and the emotionally ill before they can be sold weapons but has opposed long waiting periods and registration rules that would inconvenience all buyers.

Aquilino, a Capitol Hill restaurant owner, said that Cassidy, a former mayor of Lynn, Mass., had "all the worst instincts of a small-town mayor" and let his personal distaste for police officers interfere with NRA efforts to keep law-enforcement organizations on its side.

He said Cassidy blocked some mass-mailing campaigns as too expensive and maintained his influence in the organization by pitting sports shooting members, less concerned with gun control, against collectors and others most upset by the assault-rifle ban.

Aquilino charged in a recent newsletter that Cassidy, as the organization's top employee at its Washington headquarters, "is taking no chances that anyone will bump him from his throne on the Potomac."

Foss rejected Aquilino's charges and described him and Neal Knox, another frequent critic, as disgruntled former NRA employees "fired because they were incompetent." Aquilino said the NRA gave him a commendation when it disbanded the division he worked for.

Baker said the decline in membership in recent years might stem from a $10 increase in membership fees. He said the NRA had great success raising funds to fight the rash of gun-control measures but would have liked to do more to reach constituents of wavering legislators. "If we had an unlimited budget," he said, "we'd be doing a lot more mailing."

Wayne R. LaPierre Jr., executive director of the Institute for Legislative Action, denied another Aquilino charge that David Marshall, the former NRA lobbyist in California, had been barred from the convention for fear of provoking an embarrassing debate about legislative failures here. He said that Marshall had resigned and that NRA's new Sacramento lobbyist, Brian Judy, would be here.