MIAMI BEACH--The Democrats opened their National Convention here last night amid please for unity and a restoration of trust in American politics, but the prospect of a 20-hour session to settle a bitter credentials dispute dampened the spirit of the delegates.

Shortly before 11 p.m., Democratic National chairman Lawrence F. O'Brien ruled in favor of a role call on a challenge to the South Caronlina delegation which sought to seat two additional women. It is generally agreed here that the outcome of that rold call will be a precursor of Senator George McGovern's relative strength in the crucial California challenge.

O'Brien brought credentials committee chairwoman Patricia Robert Harris to the podium at 10:08 p.m., but her opening words following polite applause set a weary tone.

"I hope uou feel that way about 10 a.m., she said in response to the applause. And as floor managers for front-running McGovern mounted a determined effort to reclaim 151 California delegates, anti-McGovern forces dug in to buck that effort.

One dispute here centered on O'Brien's decision Sunday that the unchallenged California delegates could vote on the California question, and further, that 1450 delegate votes--or half of those delegates voting on the challenge--constituted a majority on the question.

At press time, the convention was beginning to debate on the California challenge, and everyone at Miami Convention Hall was prepared to remain here, if necessary, straight through to today's second session at which the Platform Committee's report will be presented.

The debates and voting on the early challenges moved unhindered, as Humphrey strategists laid is wait for the California challenge--what amounted to a one-shot bid to stop McGoverns. But Humphrey's managers, rocked by O'Brien's decision, were saying privately that they doubted a victory on the California question was still possible, and a loss, most observers agree, would almost surely lead to McGovern's nomination Wednesday night.

Throughout the weekend, McGovern workers and their counterparts in the Humphrey, Muskie, Chisholm and Wallace camps maneuvered for positions in anticipation of last night's challenges. O'Brien's decision was viewed as a significant victory for increasingly competent McGovern advisers; his delegate counter, Rick Steams, maintained that the Senator has 1470 firm delegate votes, which would almost assure him the California victory under O'Brien's rules.

Tension over the impending credentials fight began to mount again early yesterday. Senator Edmund S. Muskie, who has played a mysterious role of equivocation since his arrival here, called upon his fellow candidates to meet with him at 1 p.m. to work toward a compromise, thereby avoiding a convention floor debacle.

Muskie's call netted no result. Neither McGovern nor his chief adversary, Senator Hubert H. Humphrey chose to attend. Others demurred: Alabama Governor George Wallace, displaying his old fiestiness at his first extended press conference since arriving in miami was not award of Muskie's invitation.

Wallace, though gaunt, was alert as he met with the press following a meeting at which he reassured the Alabama Delegation of his intention to support the California challenge and to press for a toned down platform tonight.

He was indefinite about how much campaigning he would do this fall if not nominated, but he emphasized that he would support the Democratic nominee on "the same basis as Senator McGovern"--meaning that the Convention must measure up to his notions of responsibility.

Humphrey, for all his determined scuffing pointed toward blocking McGovern's nomination, has been unsettled by his inability to establish a firm anti-McGovern coalition. His aides still say privately that Muskie is the weakest link in the ad hoc movement; they have also become leary of the wavering Black Bloc votes.

Rep. Ronald Dellums's (D-Calif.) bolt to MCGovern from Rep. Shirley Chisbolm was the only surface shift before last night's session. His interest in upholding in another kind of coalition--one founded on the theory that McGovern is the strongest candidate at the Convention and thus has the best chance to unity the Party against President Nixon--is shared by many uncommitted delegates.

The real worry over the Black Bloc, however, is that it will hold out from either camp. Humphrey released his black delegates to Chisholm at the Black Caucus yesterday morning, a reflection of the pressure being applied by black leaders hoping to create a powerful lobby on the Convention floor.

For his part, McGovern met with the Women's Cancus yesterday in an attempt to hold its votes in line. He continues to press hard for the backing of the Mack lobby.

The McGovern staff want into planing sessions late Sunday night to map out final strategy for last night's challenges. Emerging late yesterday morning, they felt confident that they could roundup the delegates necessary to defeat not only the California challenge but others as well.

The predominant feeling was that the South Carolins challenge--the first on the schedule, brought by the National Women's Caucus--would be an early, and perhaps decisive, indicator of McGovern's strength throughout the challenges.

The McGovern staff devoted most of its efforts to tightening its communications network, which is commanded from three trailors behind Convention Hall. By late yesterday morning, a staff member had been assigned to each delegation, with the emphasis centering on pressuring Muskie, trying to woo the 275 delegates he holds.

McGovern himself is heading the effort to win Muskie, and if any deal is made, he will do the dealing.

The protest groups cut out of formal Convention activites began demonstrations Sunday night. A group of about 200 demonstrators from Flamingo Park marched outside the Playboy Plaza Hotel north of Democratic National Headquarters, then headed towards the headquarters before being turned away by a line of riot-equipped police.

The group, led by SDS but augmented by about 100 young blacks from the Southern Christian Leadership Conference campment adjacent to Convention Hall, was mostly peaceful