Two years ago, Walter Mondale couldn't wangle an invitation to the convention of the biggest union of government workers because he and his boss -- Jimmy Carter -- were considered the meanest employers since Scrooge and Marley.

But now, thanks to the bureaucracy's new three-Rs, Ronald Reagan and RIFs, many U.S. workers now view the Carter-Mondale era as the good old days.

That explains why 2,000 delegates and guests at the convention of the American Federation of Government Employes here exploded with applause when Mondale, sounding every bit a presidential candidate, launched a 40-minute verbal assault on the Reagan administration.

Mondale, now a Washington-based lawyer, said the Reagan administration is creating two Americas -- one for the very rich, one for everybody else -- by trying to convince Congress and the voters that government is too big, and that government workers are overpaid, underworked parasites.

Under Reagan, Mondale said, federal pay has been capped, pensions have been cut and the only people who have gotten the government off their backs have been "the polluters, the despoilers . . . the producers of unsafe products . . . . "

Mondale, who had a "perfect" voting record from this big AFL-CIO union during his two Senate terms, said that federal and postal employes must "get the Hatch [no politics] Act off your backs" and contribute to union PACs political action committees to counter pro-Republican, big business groups he said are "trying to buy America."

"The way to run down government is to insult everybody who works for it," Mondale told the union leaders who represent 700,000 federal workers, including about 50,000 in the Washington area.

Mondale, noting RIFs, furloughs, and the decline in federal jobs (20,000 in metro Washington since Jan. 1981), said the president has been "campaigning against all of you in this room" for more than 20 years.

"Anything he Reagan hasn't done to you that he wants to is because he works a short week," Mondale said, grinning and pointing in the general direction of California where the president is on a two-week vacation.

When he left the hall here at the Sheraton Centre, delegates chanted "RIF Reagan, RIF Reagan, RIF Reagan" for several minutes.

Their enthusiam caused a flicker of concern from hotel security officials, and baffled the New York press corps, which does not speak bureaucratese. "What is the word they are saying?" one reporter asked. "RIF, RIF, it stands for Reading is Fundamental; it is a government program," a colleague incorrectly explained.

Mondale's next stop is the American Postal Workers Union convention in Miami Beach, where he is expected to get an equally rousing welcome from a group that also once barred Carter administration aides.

The unanimous response that Mondale evoked here may be the last at this convention, which has its own national election later this week.

Dozens of challengers are seeking to unseat President Kenneth T. Blaylock, fill the vacant executive vice presidency or oust incumbent secretary-treasurer Nick Nolan.

Campaigning has been been tough and expensive. Challengers complain that the incumbents have not done enough to build up membership in the union or counter budget-cutting actions in Congress resulting in reduced pay raises, deferred pension increases and imposition next year of a 1.3 percent Medicare tax on federal and postal employes.

The election is being monitored by the Labor Department as part of a dispute that grew out of the 1980 convention in Honolulu, when challenger Carl K. Sadler claimed that many delegates voting for their locals had been selected improperly. After an investigation, the Department said it did find many irregularities but refused to order a rerun of the election in which Blaylock narrowly defeated Sadler, the union's onetime chief lobbyist.

Whoever wins, it is clear that President Reagan has lost the support of his employes -- if the ones here this week are at all representative of the mood of their 2.8 million co-workers who stayed home this week to mind the store.