SAN JOSE, COSTA RICA -- This country's forceful young president, who will meet in Washington Tuesday with President Reagan, is doing what no other Central American leader has been able to do since the Reagan administration began financing the counterrevolutionary war against Nicaragua's leftist government in 1981.

To the chagrin of many administration officials, 46-year-old President Oscar Arias has staked out a position opposing both the Sandinista government in Managua and the U.S.-backed rebels fighting them, and he has drawn all the region's other governments toward that position.

Arias is the principal author of a Central American peace plan signed in Guatemala on Aug. 7 by all five Central American presidents that has since provoked increasing hope in the region and concern in official Washington, where a number of Reagan administration officials and conservative lawmakers view it as favoring the Sandinista government at the expense of the contras.

In a recent interview here, Arias denied with a resolute shake of his head that he is locked in a confrontation with the Reagan administration over Central American policy, and he bristled at the suggestion that Washington ever pressed him to back off from his plan.

Despite Arias' denials of confrontation with the White House, his plan is widely viewed as imperiling a proposed $270 million in new U.S. military aid for the rebels, which is a Reagan administration priority.

Arias, now on an eight-day visit to the United States, also will speak to an informal gathering of lawmakers in Washington Tuesday. He addresses the United Nations Wednesday.

"As long as Washington supports the contras, it will be isolated. No other country in Central America supports Washington on that," Arias told reporters Monday in Kansas, where he participated in the Landon Lecture Series at Kansas State University. He said the administration could derail the accords by insisting on new contra aid, according to press accounts.

Before Arias took office in May 1986, Costa Rica, one of Washington's most economically dependent allies in the region, also was one of its most pliable, harboring the rebels while publicly professing neutrality.

But Arias conceived the first peace formula that bolsters all five presidents by highlighting their common political interests instead of accentuating their disputes. By calling for a region-wide cease-fire and an end to outside aid to rebel armies, the plan also distances the presidents as a group from the Reagan administration.

Sen. Christopher J. Dodd (D-Conn.), who visited three Central American countries, including Costa Rica, over the weekend, called Arias "the man everyone else is talking to" about the peace plan.

Arias is gambling that giving Costa Rica a high-profile international peacemaker's role also will cement his domestic support. So far it is working.

A poll of San Jose residents taken by the Gallup affiliate here shortly after the signing of the peace plan showed a 16 percentage point increase in Arias' popularity, raising his approval rate to 71 percent. The poll found that just under 90 percent of city-dwelling Costa Ricans believe that the plan is good for their country, which borders Nicaragua.

Arias' domestic opponents charge that he is too busy tending to his international image and is neglecting Costa Rica's ailing economy and $3.7 billion foreign debt. This month, Arias failed to guide his sorely needed emergency tax package past an assembly deadlock.

In an unusual twist in a country long accustomed to interference from Washington, his opposition in the assembly tried to block Arias' trip to the United States, arguing that he is interfering in U.S. internal affairs.

White House officials have known Arias would be trouble since they summoned him to meet with President Reagan in Washington while he was on a private trip to the United States in mid-June. According to a Costa Rican official who was present at the meeting, Arias launched into a 35-minute monologue, telling Reagan that the contras were "not doing the job" against the Sandinistas.