Last month, Tom Hayden, former antiwar activist, husband of Jane Fonda and unsuccessful candidate for The U.S. Senate, took over the government of the city of Santa Monica.

Hayden plays down his role. "I've never been inside the Santa Monica City Hall," he said. But despite his protestations, Hayden, to a great extent, is the Campaign for Economic Democracy (CED).

And this grass-roots organization took over the Santa Monica city government in last month's elections.

A CED-backed coalition, Santa Monicans for Renters Rights, elected four city council candidates, a member of the rent board and two members of the board of education. With one CED incumbent already on the city council, the CED now holds a 7-to-5 majority, and can appoint both the mayor and vice mayor.

Historically, Santa Monica has been mostly a working-class area, but, like other clean-air communities, it is getting richer.

The Haydens, for example, bought their modest Santa Monica home in 1973 for $40,000. Today, the house next door is on the market for $400,000. As affluence comes to Santa Monica, it is bringing with it younger people and more liberal politics.

Founded after Hayden's Senate loss in 1976, the CED is supported mostly by donations from Fonda and other Hayden supporters. And the 10,000 or so dues-paying members, including dedicated volunteers who work in CED-endorsed campaigns, are largely attracted by Tom Hayden.

There are mumblings about Hayden's being an impure ideologue, and accusations of opportunism. But few are willing to criticize him publicly.

A state legislator from the Los Angeles area explained the reluctance to talk about Hayden this way:

"Tom Hayden represents an organization that has political skills and political potential. So, particularly if you view yourself as having statewide potential, when he has this statewide network behind him . . . you are going to be wary and respectful and careful not to alienate him.

"On the other hand, anyone political runs a risk being associated with Hayden. In some districts, he is still politically untenable."

In politics, particularly local politics, warm bodies are power. The power generaged by the energetic CED volunteers flows directly to Hayden.

Out of 1,600 delegates to January's Democratic state convention, 320 delegates and 90 alternates were CED members, elected because the CED sent 2,000 of its members to delegate-selecting caucuses in 65 of the state's 80 assembly disricts. Hayden was the only non-elected official asked to chair a portion of the convention.

Hayden and the CED choose issues wisely. In Santa Monica, for instance, the big issue was rent control. It is no coincidence that a little more than 80 percent of Santa Monica's residents rent.

"Civic participation," Hayden said, "is potentially the most important thing. Community control. Rent control. Crime control. Toxic wastes control."

Hayden was pleased by the recent move by Continental Airlines employes to try to purchase 51 percent of their company. That's economic democracy in action, and it's something he thinks there will be more of through "the malfunction of corporations."

Economic democracy, Hayden insists, is not socialism. "Socialism means the expansion of the public sector in the form of nationalization. I question whether that's the good life or an expansion of bureaucracy."

He is a bit more vague in his description of his philosophy:

"Economic democracy is less concerned with the form and more concerned with the content: is the bureaucracy responsive? No. Is the University of California responsive? No. Is it accountable? No. Is the department of water and power responsive? No. Does it put consumer interests first? No.Does it have a humanized workplace? No."

What is not included in Hayden's definition is how his philosophy would change those "nos" the "yeses."

Hayden is not shy about his contempt for the Reagan administration, and neither is he hesitant, these days, to express support for that old establishment institution, the Democratic Party.

He said recently that his biggest current concern, "both in the state and nationally, is the future of the Democratic Party." When the metamorphosis of Tom Hayden, radical, to Tom Hayden, party regular, is questioned, he responds, "The Democratic Party was tight as a drum in 1968. The party has opened up. If you bang on a door and the door opens, you don't keep banging, you go in."

Hayden consults not only with his frequent political ally, Gov. Edmund G. (Jerry) Brown Jr., but also with officeholders such as Sen. Alan Cranston and Los Angeles Mayor Tom Bradley, both Democrats.

Most observers expect Hayden to take his second electoral plunge in 1982. He has been rumored to be a candidate for just about as many offices as exist in California, but the current best guess seems to have narrowed to either a congressional or state assembly seat.

Mickey Kantor, a Democratic Party strategist and law partner of Democratic National Committee Chairman Charles Manatt, said he believes that Hayden's background may make him, for the time being, unelectable on a statewide level, while the same would not hold true in a run for Congress or the assembly.

"As soon as he is elected to an office," Kantor added, "I think a lot of the problems people have with him will go away. They'll find him reasonable and able to get things done."

Hayden recently shared a dais with Cranston, Bradley and former governor Edmund G. (Pat) Brown Sr., at a "roast" to honor Cranston's retiring press secretary, who is now working part-time for Hayden.

When it was his turn to speak, Hayden explained why he'd hired the press secretary: "Anyone who could turn Alan Cranston from a commie one-worlder into a United States senator is the kind of man I can do business with."