COMBAT IN PANAMA : Finally, Opposition’s Endara Gets His Chance : Panama: The new president, who won election by 3-1 margin, has little government experience.

Last May, Guillermo Endara, a rotund, affable Panamanian lawyer, ran for election as president of Panama against the hand-picked candidate of Gen. Manuel A. Noriega. Independent exit polls showed he won by a margin of 3 to 1.

On Wednesday, after more than seven months, Endara was sworn in--by a Panamanian judge during post-midnight ceremonies at a U.S. military base as American troops were launching their military drive against Noriega. The hurried, furtive nature of the swearing-in demonstrated the problems Endara and his government will confront.

“There are a number of things that this government has to do, given a chance to form itself,” Secretary of State James A. Baker III said at a news conference Wednesday. “. . . It will be taking some actions to form itself, just as it would have had to do had it been permitted to assume power fairly following a free election on May 7. So these things still have to be accomplished.”

Until now, Endara has had little experience in government. He is a former law professor and the leader of a small group of opposition figures who, with the support of the United States, have been willing to challenge Noriega.

Endara, 53, spent most of his political career as the aide and spokesman for another veteran Panamanian opposition figure, Arnulfo Arias Madrid, who died in Miami last year.

Arias was elected president of Panama three times and each time was overthrown by the Panamanian military. Over the past two decades, Arias had sought unsuccessfully to challenge the military power of Panama’s two leaders, first Gen. Omar Torrijos and then Noriega.

Endara’s parents also were associated with Arias. When Endara was only 7, he went into exile with his family after Arias was ousted from power for the first time.

After graduating first in his law school class at the University of Panama, Endara served for two terms in the Panamanian National Assembly. In 1968, when Arias became president once again, Endara served as his minister of planning and economic policy. It was not a steady job: Arias’ government was overthrown within two weeks. Endara was jailed and eventually fled the country.

In recent years, Endara taught law and worked as a partner in a Panama City law firm. When opponents of Noriega selected him as a candidate for the presidency early this year, Endara acknowledged that his ties to Arias had been a crucial factor.

“I know I wasn’t chosen so much for my own personal merits but for my ties to Dr. Arias,” he said in one interview.

When it became evident that Endara and his Civic Democratic Opposition Alliance had won the election, Noriega nullified the results. Three days after the polling, members of the so-called Dignity Battalion controlled by Noriega beat up Endara and vice presidential candidate Guillermo (Billy) Ford.

Endara, who was clubbed with an iron bar, wound up spending his 53rd birthday in a hospital bed.

During the campaign, Endara and his party promised to revive democratic institutions in Panama and to purge corrupt elements in the Panama Defense Forces. Most of all, Endara said, his first task as president would be to seek to remove Noriega from power.

“If, on the 1st of September, 1989, it has not been possible to arrange Mr. Noriega’s departure from the command of the defense forces, I will have no choice but to remove him,” Endara said last spring.

Those efforts failed. And early Wednesday morning, U.S. troops took over from Endara the job of removing Noriega.