In Washington, it is gross to talk about anything except what Ronald Reagan is talking about.

Presidents control the nation's agenda in the nature of things. But no one has had greater success in controlling the conversation as well.

It's as if there were a giant blackboard on the edge of the Potomac, on which Reagan writes "the theme of the week" -- the only acceptable topic.

Thus over the weekend, just before a series of votes that could determine the future of the planet, the hemisphere and the skies above, all the movers and shakers were dutifully wading through the muddy waters of Reagan's tax-overhaul proposal.

Congress, in both houses, is confronted with cosmic issues: the arms race, the continued observation of the SALT II pact, "Star Wars" and aid to the rebels in Nicaragua.

These issues did not merit so much as a nod on Sunday's talk shows. Panelists obediently chewed over the morality of ending the deductibility of tickets to sporting events.

A hangover worse than any caused by the newly imperiled three-martini lunch could smite the country when the consequences of decisions on the cosmic issues start coming in. But there was Treasury Secretary James A. Baker III solemnly detailing the "national security" implications of the oil and gas depletion allowances.

On David Brinkley's show, reporter Sam Donaldson badgered New York Gov. Mario M. Cuomo (D) to come up with his own tax proposal. Donaldson insisted that Cuomo present a "package" of tax reforms, something the governor complained it had taken the president some seven months to prepare.

That is another tyranny imposed by Reagan on the city he loves to hate. If you oppose what he proposes, you must come up with a "positive alternative," preferably a "compromise package," which implicates you in the scheme no matter how nutty you found it to be in the first place.

The president is erasing the word "no" from the dialogue. In Reagan's Washington, it has become an expletive deleted.

Take Star Wars. Many eminent scientists, including some 50 Nobel laureates, have said the antimissile defense plan will not work. It is the wrench in the works of the Geneva arms talks, a wedge between us and our allies. It would cost $1 trillion and add to the federal deficit -- a topic outlawed by Reagan -- but nobody says, "Get this thing out of here."

Instead, senators are trying to cut his request from $3.7 billion to $1.6 billion. Sen. John Kerry (D-Mass.), a freshman unlettered in local folkways, wants to freeze funding for Star Wars. He is considered foolhardy and "unrealistic."

On aid to Nicaraguan rebels, Reagan pounded his fist on the table in frustration. He is outraged that Congress refused to provide military aid for what he calls the "democratic resistance," despite the large number of former Somocistas and adventurers in its ranks. He could hardly believe it when the House refused even to give him $14 million for "humanitarian aid" for the forces, which are supposed to prevent the ruling Sandinistas from invading Texas.

No one beyond the most possessed redbaiters in the Senate believes that the contras can win, with or without U.S. money, but people are scurrying about preparing "compromise packages," grappling with whether trucks are "nonlethal" assistance.

"Reagan is right that it's micro-management of foreign policy when you hear people haggling over whether it should be two jeeps or four," said Sen. Christopher J. Dodd (D-Conn.), who wants to end everything but help in relocating the contras, or counterrevolutionaries. "But once you start doing that, he's gotcha -- you're arguing on his turf."

The House immediately realized that it had committed the unforgivable Capitol crime when it said no to Reagan on all aid.

It is preparing to make amends. Daniel Ortega, the president of Nicaragua, gave members a handy excuse by going to Moscow. They were secretly grateful to him. Now they can say yes, which sounds so much better, even though it could lead to the involvement of U.S. troops.

Also, Reagan may scrap SALT II, even though the Soviets would pay much more than the United States would if it were continued.

But the right-wingers, who claim that the Soviets are cheating anyway, want to uncap the arms race. And Reagan, the man who wants to make nuclear weapons "obsolete," is listening to them.

In Washington, it is written somewhere that you cannot look at the forest. You must see only the trees, which are all owned by Ronald Reagan.