Cardinal Jaime Sin, who frequently in the past has assailed the rule of Philippine President Ferdinand Marcos, has moved into a new dual role that casts him as an alternating presidential confidant and critic as he seeks to find a peaceful reconciliation of opposing forces in this tense country.

On Thursday night, the cardinal's antigovernment rhetoric reached a new pitch of abusiveness, when he called Philippine elections a "travesty" and likened Marcos' suppression of the media and record of evasiveness to Nazi officials. Yet last night, he met with Marcos to offer a national reconciliation plan that is "the last feasible alternative to avoid the violent confrontation and bloody revolution made imminent by the temper of the times," according to a statement released by one of Sin's aides.

The cardinal's meeting at Malacanang Palace yesterday with the president, a church spokesman said, was in the role of a "parish priest visiting with a parishioner."

The cardinal's dual role is one of the strange wonders of Philippine politics, a phenomenon difficult for outsiders to understand. He is at times the church militant scouring the demons of temporal government and at times the benevolent adviser pictured chatting amiably with the smiling president.

Whatever his secret to success, it has made Sin into a figure of towering importance in a country torn by violence and recriminations. Some regard him as the strongest source of restraint on a government often given to extremes. Since the assassination a month ago of opposition leader Benigno S. Aquino Jr., the cardinal has emerged as perhaps the only person capable of putting things back together.

"He is a political bridge between the president and the opposition," said one western diplomat who has observed the change. "He has stepped up his pressure on Marcos , and the church is going to be more politically active after this."

The cardinal has such a preeminent political position, in part, because no one else commands much broad respect. In the post-Aquino period, the opposition suffers again from factionalism. It has many loud voices proclaiming themselves to be leaders but has no one who can summon a national constituency.

Sin's political role has changed considerably since Aquino's death. Before, he was a sporadic critic, speaking out on occasions of government excesses, such as rigged elections. He has become a more deliberate planner and is trying to build a consensus around his ideal of "national reconciliation." He first offered the plan in February, but the palace ignored his proposal.

A copy of Sin's proposal shows that he envisions a council composed of representatives of the government, church, opposition political groups and private business. "Each member must be generally acknowledged as being of unquestioned integrity and enjoying a high degree of respect and credibility," the proposal declares.

The cardinal's plan lists several "pre-conditions" for reconciliation--free elections, a free press, an independent judiciary and a "thorough and impartial" public investigation of Aquino's assassination.

Although he has spurned the proposal in the past, Marcos may be more inclined to listen now that his political base and credibility have been so badly eroded by the assassination of his one-time enemy. But last night, according to a church spokesman, the president promised only "to take a look at" the proposal.

It was to Sin, the leader of 45 million Philippine Roman Catholics, that the United States quickly turned after Aquino's assassination, sensing a debacle ahead in plans for President Reagan to visit here in November.

A meeting between Reagan and the cardinal was arranged for the visit to soften the spectacle of an American president half-way-round the world meeting solely with the head of the government that is widely suspected of having Aquino killed.

It is not likely that pictures of a Reagan-Sin meeting will satisfy the opposition, which views Reagan's planned visit as one more symbolic American gesture of support for Marcos. Demonstrations are planned and at least one prominent businessman here has advised the U.S. Embassy to have the trip canceled.

But U.S. officials say they believe that the cardinal's enormous moral influence may take some of the sting out of the criticism and lend a semblance of impartiality to the visit.

Recently, Sin has been meeting privately with a group of influential businessmen and academicians, many of whom have turned into quiet critics of Marcos since the assassination. The group is described by one participant as composed of people "seeking an outlet" for their anger but not prepared to join ranks with what they regard as the opposition's ineffectual leaders.

"There are a number of private groups which are preparing to line up behind the cardinal," the businessman said. "We all see the need for some cohesion and the cardinal will wind up at the head of this."

What they are lining up behind is the cardinal's vaguely spelled-out plan.

Moreover, for the next four weeks the idea is likely to lie dormant because the cardinal will be in Rome for a planning meeting with other church leaders.

The idea of a "council of reconciliation" has not aroused notable enthusiasm among the regular opposition leaders who do not believe it is possible to negotiate with Marcos and who insist that he must resign. Their slogan is "resignation before reconciliation," an indirect rebuke of the cardinal's plan.