MANCHESTER, N.H. -- With a thousand Democrats at St. Anselm's College ready to acknowledge Sen. Paul Simon as the new rising star, he flubbed his golden opportunity.

The performance of the six candidates last weekend was close enough for a six-way tie. But with higher expectations, Simon was the loser. Party activists, who came to the state convention suspecting the 58-year-old unabashed liberal from downstate Illinois might just fill the vacuum, left still looking.

The biggest loser was the party itself. Brief candidate speeches were spliced with fruitless efforts by State Democratic Chairman Joe Grandmaison to clear aisles and remove obstructing placards. A nationwide live cable television audience got two hours of noise, confusion and candidate competition in bashing Ronald Reagan. While leaders publicly praised the exuberance, one campaign manager told us: ''This is what's wrong with our party.''

As delegates gathered here, interest was rising in Simon's playing Cinderella. Gov. Michael Dukakis of neighboring Massachusetts enjoys total name identification and a huge lead in the polls. But like previous front-runners Edmund Muskie in 1972 and Walter Mondale in 1984, he is threatened by soaring expectations and restrained enthusiasm.

Simon's ''15 minutes'' of fame was at hand. Even supporters of other candidates acknowledged his poll-leading surge in Iowa was being duplicated here. Lacking delegate strength, he brought in college students to make noise. With several delegates undecided between him and the Rev. Jesse Jackson as their first choice, Dukakis may be outflanked on the left. Several told us they loved to listen to Simon's basso profundo voice on the radio.

To the surprise of party leaders, Simon's rise has not been limited to activists. Cambridge Reports, Inc., released a poll the day of the convention showing Dukakis -- over 50 percent in past polls -- dropping to 37 percent and Simon rising from invisibility to second with 14 percent.

Consequently, as Democrats convened Saturday afternoon, word passed around the floor that Democratic National Committeewoman Patricia Russell -- adrift since the fall of Sen. Joseph Biden -- was going to Simon. That signaled possible movement by orphaned Bidenites here matching their Iowa cousins.

''Not true,'' Russell told us just before Simon spoke. On the contrary, just the other night she and some 25 other party activists discussed options and, even this late in the game, stayed neutral -- a widespread phenomenon this year.

Nothing happened the next two hours to win Pat Russell's heart. Leading off by luck of the draw, Simon seemed unnerved by his newfound celebrity. With each candidate limited to 2 1/2 minutes to leave time for a forum that never took place, Simon started a bad precedent by droning through nine uninspired minutes of shards from his basic campaign speech. The senator's voice, rich on radio, was a thin reed in the raucous atmosphere.

If delegates later could not remember what Simon said, it was no wonder. He knocked ''those who criticize New Hampshire and Iowa'' for being unrepresentative, then declared: ''I want a government that cares.'' Responding to sniping over his support for new spending programs and a balanced-budget amendment, he called for ''old-fashioned, Harry Truman pay-as-you-go government'' and appealed finally for government that ''cares,'' ''believes,'' ''dreams'' and ''acts.'' Unimpressed, one veteran insider went home to watch Simon on tape. ''It didn't get any better,'' he told us.

The good news for Simon was that his opponents were not much better. Bruce Babbitt's message was most coherent but least congenial, calling for higher taxes and lowered spending. Rep. Richard Gephardt was vigorous but shrill, proclaiming ''the values of this country are selfishness and greed.''

Dukakis was most faithful to time constraints with a five-minute message, memorable only for charging Gephardt with stealing ''my line'' that calls for ''Star Schools'' instead of ''Star Wars.'' (Actually, the line is Sen. Edward M. Kennedy's.)

Sen. Albert Gore Jr., fearful of arousing catcalls from party activists, ignored his campaign posture of moderate among liberals and, with a shout, joined the Reagan-bashing. As shy of delegates as Simon, Gore imported a cheering claque weighted with well-dressed Tennesseans.

The only enthusiastic response beyond his own cheering section went to Jesse Jackson, along with congratulations from Russell. In 15 minutes, Jackson brought delegates to their feet by proposing a budget decimating defense and taxing corporations and the rich.

But voters here know Jackson cannot win New Hampshire or get nominated. More significant was the absence of yearning for New York's Gov. Mario Cuomo. Democrats here accept his ''no.'' They believe their nominee will come from the six who bellowed at them last weekend. That means Paul Simon may yet fill the vacuum even if he flunked his ''15 minutes'' of fame.