What may be remembered as "The Battle of Washington" was launched yesterday in an overcrowded banquet room of the Ramada Renaissance Hotel. More than 260 delegates came to the 15th annual conference of Opera America, ostensibly to deliberate on "The Future of Opera" but more urgently to fight for that future. "We want to get a message across to Washington," said Martin I. Kagan, executive director of Opera America.

The target of speakers in the conference's first day of public sessions was the Reagan administration plan to reduce funding for the National Endowment for the Arts' grants to opera and musical theater. At meeting's start, the proposed budget had not yet been made public, but various leaks and rumors put the planned cuts at figures up to 33.3 percent. In fact, it was learned later that the proposed NEA cuts total 11.7 percent.

By no coincidence, the guest of honor at yesterday's luncheon was Rep. Sidney R. Yates (D-Ill.) whose subcommittee will be the battlefield where proposals for that cut are thrashed out.

Yates was hailed as a fan who has collected opera records since he was 12 years old and was presented an engraved cube of Steuben glass inscribed to "Sidney R. Yates, our standard-bearer."

"I would have thought Opera America would prefer me as a spear-bearer," Yates quipped. Then he picked up the spear. "The president proposes, the Congress disposes," he said. "The Congress, in recent years, has been much more friendly to the arts than Mr. Stockman."

When the leaks on the NEA budget cuts began to appear, he said, "I suddenly became very popular." He recalled a conversation with Martin Feinstein, general director of the Washington Opera, in which he said, "Martin, I told you you should have offered Frank Sinatra the lead in 'The Marriage of Figaro,' " and Feinstein replied that he had tried to cast the Beach Boys in the opera as a quartet."

Earlier, in a keynote address, music critic and financial writer Martin Mayer noted that for most American opera companies the current NEA grants amount to about 3 percent of their annual budgets. "That's peanuts," he said, "but for most of you it's the biggest peanut you have." In England, he said, the government subsidizes opera at a rate of about $22 for each ticket sold. In the United States, the comparable figure is about $1.25 per ticket.

In spite of these figures, he said, opera is probably more popular in the United States today than it has been at any time since the age of Caruso, and the artistic standards have risen to a remarkable level. "The chances of a good evening at the opera are now better here than in Europe," he said. "There are a good dozen American cities where the average level of performance is higher than in all but the three or four best European houses."

It should be possible to maintain those standards, Yates told the conference: "I believe Congress will help you in a reasonable manner. I don't think you will have to reduce the sextet in 'Lucia' to a duet."

"Sid Yates was my congressman when I lived in Chicago many years ago," said one Washingtonian at the meeting. "From what I know of Chicago politics, I'm probably still voting for him."

He seemed pleased at the thought.