BOSTON--When Jerry Brown was governor of California from 1975 to 1983, he slept on a mattress on the floor of a $250-a-month apartment rather than live in the spacious new governor's mansion built under the administration of Ronald Reagan.

The move was designed to show a hatred of the establishment. And that anti-establishment attitude has not changed.

Now, Brown runs a barebones presidential campaign that accepts no contributions greater than $100, camping out in supporters' living rooms rather than the hotel suites frequented by his rivals.

And as the primary season enters a crucial stage, there are signs that Brown's anti-establishment message is catching on--and that his bare-bones campaign may be adding some flesh.

"Raise your had if you've never given a thousand dollars to a presidential candidate," Brown asked a crowd of more than 1000 longtime supporters, new converts, and lookers-on yesterday in the Boston Common.

When a sea of hands went up, Brown said, "You are the kind of people successful presidential candidates do not spend enough time with."

Brown has spent nearly all of his time on the campaign trail railing against the political establishment. Still, the successful campaign that he appears to be building could make Brown a Democratic contender to be reckoned with.

Less than a week after finishing fifth in New Hampshire, Brown used his opposition to nuclear power to secure a virtual tie with former Massachusetts Senator Paul E. Tsongas in Maine. In the pas week, the former California governor has won the hotly contested Colorado primary and the Nevada caucuses.

In spite of these successes within the system, the candidate says he is committed to complete reform of the current political structure.

"This is a system here I'm trying to change," Brown said yesterday. "If you feel you're being ripped off, lied to, then this is your candidacy right here."

Brown's strongest supporters say that it is this particular aspect of the candidate's message that appeals to them most strongly.

"I've listened to liberals complain about the corruption of American government all my life," said John C. Cole, 28. "And I finally found someone who's going to do some-thing about it--if he gets elected."

Cole carried a sign that read "Down with PACs, Up With People," a reference to the political action committees that fund many top political campaigns but have been rebuffed by Brown as well as his Democratic rivals Tsongas and Arkansas Gov. Bill Clinton.

With his populist rhetoric, Brown's critics have wondered aloud if the former chair of the California Democratic Party, whose father was also governor, is the right man for his message.

Brown acknowledges his political history, but maintains that his association with the system will help him overhaul it.

"It's not that I don't know how to [raise money]," Brown said during the rally yesterday. "I've raised more money that Clinton or Tsongas ever have, but I know it doesn't work."

"This country has been presided over by leaders who have forgotten who they work for," he said.

Beyond his fierce opposition to the system, Brown emphasizes his stands for environmental protection and energy efficiency and against nuclear power.

"We put a space shuttle up there, we've got nuclear bombs--so we can build a car that won't pollute," Brown said.

Brown has also been known to adopt policies of his rivals on issues that are not his focus.

The candidate openly endorses the health care plan of Nebraska Sen. Bob Kerrey, who dropped out of the race for the Democratic nomination last week.

And Brown has joined Tsongas in criticizing a Clinton-proposed tax cut for the middle class.

"I'm not for a middle class tax break like Clinton wants...that is only a dollar a day," Brown said.

As the Democratic race has narrowed to three candidates, Brown and his message, once ignored by the media and derided by commentators as the product of too many meditation sessions with his Indian gurus, is now considered mainstream.

The candidate was featured on the front page of yesterday's New York Times, and the newspaper defended his proposal for a flat, 13 percent tax rate on its editorial page.

And while so-called "Governor Moonbeam" was ignored by the other candidates in early debates. Brown now receives respect and sometimes harsh condemnation from his presidential peers.

The attention Brown now receives is commensurate with his political clout after respectful showings in the season's early primaries.

But with Democratic voters concerned about nominating a candidate who can win the presidency, even those who agree with Brown say they are not sure they will vote for him.

"I think we just need a complete change," said Rita Carson, 52, who works in a downtown Boston bank."But I worry that he's not electable."

"Still, I'd vote for Jack the Ripper over George Bush," Carson said.