In the 20 years since its birth, the Institute for Policy Studies, a leftist think tank, has enthusiastically weathered conspiracy indictments, telephone surveillance and the distinction of winning a place on the White House "enemies" list during the Nixon years. The FBI even rummaged through its garbage one day.

But last night IPS founders Richard Barnet and Marcus Raskin might have faced their toughest go-around yet. They were to be roasted by some of the loudest liberals of our time in commemoration of IPS' 20th year.

But instead, it was Raskin in the end who stole the show by roasting his roasters. He said he was going to ask Rep. Ron Dellums (D-Calif.) to address an IPS seminar on how to dress down in a depressed economy.

Poking fun at George McGovern's languid pace in choosing a running mate in 1972, Raskin quipped, "Ralph Nader was his 27th choice--before Bob Dylan and after Abbie Hoffman."

Raskin also added that "Ralph has seen 'Gandhi' five times trying to figure out how to get all those millions of people to work for free."

In the true spirit of political Washington's ability to laugh heartily at itself, the other roasters tried. But as former SALT negotiator Paul Warnke joked, the likes of McGovern, Dellums , author Rita Mae Brown and Nader "threw marshmallows and bouquets."

Nearly 1,000 die-hard liberals assembled at the Old Pension Building for an indoor picnic, not unlike the rallies of the late '60s. It was in sharp contrast to the uptown Republican parties of the Reagan administration.

For $35 a head, everyone ate barbecued chicken and ribs from paper plates. Each table got one carafe of wine and one dish of a cake-and-whipped-cream concotion for dessert. The floor throbbed with bluegrass sounds, and only waiters wore tuxedos.

The most talked about no-show was Bianca Jagger, who has been associated with IPS through her volunteer work for Salvadoran refugees. Guests craned their necks at the door, and organizers kept calling her New York hotel. But the so-called roast had to soldier on without her.

Cora Weiss, whose father, perfume mogul Samuel Rubin, heavily funded the institute, joked, "Let's face it. The reason that Marc Raskin and Richard Barnet are where they are is because they couldn't hack it as lawyers."

When it came her turn, Rita Mae Brown said, "I was a woman fellow at IPS 10 years ago--a time of subpoena envy . . .

"Also, it was a time when Cuba was a summer camp for the New Left."

Entertainer Harry Belafonte produced a letter from the CIA to filmmaker Saul Landau, an IPS fellow, who had sued the CIA over tampering with his mail. The letter, which was accompanied by a $1,000 settlement check, encouraged Landau to get in touch if the agency could be of any additional help.

"The most important point of this letter," said Belafonte, waving it to the throng, "is that through the years, we will be in touch." Everyone roared.

IPS was a child of the New Left born during the civil rights era and weaned on the Vietnam war, when the Left's bloodline was nourished by attacks from the right. The idea was to create a respectable policy center for the left, one in which liberals, as visiting and resident fellows, would supply the necessary political research. IPS started with six fellows and $200,000; today it has 35 fellows and a working budget of $2 million.

"In the fall of 1963 it was the 'We Shall Overcome' period," said Raskin. "We felt that the problems could be worked out together and we could fight for civil rights. It was a good feeling of citizenship. There are many things that we've accomplished: civil rights, human rights and women's rights. We've won.

"But the sad thing lately is the Reagan administration's attempt to take back some of those gains. They've really tried to be counterrevolutionary in a way, but I don't think they'll succeed."

Despite its disenchantment with the status quo, this is one organization that is not encouraged by the prospects for real change in 1984.

"No individual candidate can make a difference until the Democratic Party adopts an alternative vision of what the country can be," said Barnet. "No. Up until now, I'm not very optimistic about 1984."