It wouldn't have been a Democratic convention without the purposefully scruffy radicals shouting Marxist slogans, the angry black activists, the bemused cops behind the johns and hookers United Against Repression, the animal rights people sweating in gorilla suits, the gay rights people, the antinukes and all the other perpetually earnest and outraged souls who marched their message out again today.

Thousands of them converged with a righteous zeal on the Madison Square Garden convention site here in a confrontation that has become over the years as much a tradition as the insurance executives and school-teachers n funny hats on the convention floor.

But a few blocks uptown, at the old Roseland Ballroom, a group called ACORN hosted a different sort of counter-convention -- one that presented, a more pertinent picture of the polictical realities of 1980 and the relationship of the Democratic Party to its kaleidoscope of constituencies. ACORN, for Association of Community Organization for Reform Now, set out this year to elect poor people as delegates and it did -- 22 delegates and 32 alternates.

One of the largest and most visible of the growing number of grass-roots organizations clamoring for a voice in party affairs, ACORN is short on radical rhetoric and long on pavement-pounding in local communities. Members spent years attacking bread-and-butter issues, such as utility rates and sales taxes on medicine, before sallying forth this year for the first time into presidential politics.

When 1,100 low- and moderate-income members of ACORN gathered today under the glitter-globes on the renovated Roseland dance floor, it was not so much to condemn the system as to celebrate their modest initial victory within it.

What they are after this year, they had decided, is greater representation for the poor within the party that prides itself on representing the interests of the poor.

At the 1976 Democratic convention, they estimated, only 6 percent of the delegates had incomes under $10,000 while 44 percent of rank-and-file Democratic voters earned less than $10,000.

A Washington Post survey of Democratic delegates in 1980 makes a similar point. Like the Republicans, the Democrats at this convention are far more affluent than the public at large. Two of every three are college graduates, and one-quarter of them report having family incomes of more than $50,000 a year.

"I'm tired of the fat cats doing the decidin' and the dispensin'," growled Lois Groves, a formidable black woman from South Philadelphia who came here on a bus and is staying nights in a dormitory at Fordham University in the Bronx with several roommates, sharing a cut rate. She earns a modest income, she said, taking care of other people's children and has a family of 12 of her own to support.

What ACORN gained this year is a commitment from the Democratic Party to form a commission to study ways of making it possible for "more members of the party's low-and moderate-income majority to become delegates." The convention is expected to approve the proposal without controversy.

One of the questions ACORN wants the commission to look into is how to provide financial support to low-income people who could not otherwise afford the high costs of party politicking and travel to conventions.

The organization, which represents 30,000 low-and middle-income families in 20 states, began in the snows of the early primary and caucus states and worked to influence the electoral process.

Its members played a visible, if not decisive, role in some key caucus and primary states, most notably Michigan, where they joined the United Auto Workers to give Sen. Edward M. Kennedy a narrow victory. And its delegates span the spectrum of involvement at the convention.

Sidney Bass, an unemployed transit worker, was elected in the 13th Congressional District in Michigan as a Kennedy delegate. Boonie Skaff, a warehouse clerk from Sioux Falls, S.D., is a Carter delegate. Paula Stewart, a health clinic receptionist from St. Louis, is uncommitted.

Many in the ACORN group, which includes young, old, blacks, Hispanics and whites, sold cookies, held gumbo dinners or disco dances. Some sold ads in their convention program book to local unions or merchants to finance their trip here.

Mona Lisa Thomas, 12, who came here from South Philadelphia with her mother and cousin, stared up at the liveried waiters in the windows of "21" who stared down at her crowd as they marched toward a hotel meeting with their state delegation. Asked if she considered herself poor, she said no. "But a lot of people I know are. We have a lot of old houses that need to be built over," she said; holding a big poster she had drawn with an acorn on it.

The kitschy atmosphere of faded romance at Roseland seemed incongruous at first glance for a counter-convention of poor people.But the group's message was appropraite to the place -- the more things change, the more they stay the same.

For all the to-do in the Democratic Party over quotas, and this and that disadvantaged group and reforms, ACORN's founder Wade Rathke said, "To say the fat cats are still running the party is not inaccurate.

"Sure there are more women, and more blacks and other minorities represented. But they are not necessarily poor women, poor Chicanos, or poor blacks. They are the opposite, in fact."

Rathke, like many of the ACORN organizers, is a veteran of the civil rights movement of the Sixities, retrenched firmly in the grassroots, using his old organizing skills a new way.

Rathke called all the talk of the decline in the power of the party "bull."

"You can talk about the influence of the media versus the party system" and other fine points, he said, "but that's not the perspective from the bottom up. To someone who is powerless, the party still looks like it has some power."