THIS WEEK in Washington, being an optimist means you don't think the invasion of Grenada was a dress rehearsal for Nicaragua.

If Grenada goes down well with the shell- shocked voters, the conquistador in the White House may just decide to throw off the veils of his "secret" war against the Sandanistas and send in the Marines to overthrow another government that gets on his nerves.

"Restoring democracy" there could take a little longer than in Grenada, since the amply forewarned Nicaraguans maintain a standing army of some 25,000 and a militia of equal size, which Reagan says proves his point that they are going to invade their neighbors.

But our nervously elated war planners may feel that they have flight-tested the most important thing, that is the public relations of invasion.

By the Grenada formula, first, you get the neighbors to complain. That was a cinch with the infinitesimal islands surrounding Grenada. Perhaps a mere mention of the new funds available to "good" countries in the Caribbean Basin Initiative made them see that they could not live another day with the Marxist thugs on Grenada.

The same operation would be a piece of cake in Nicaragua. Honduras, El Salvador and Guetamala, who are in hock to us for more arms than they know what to do with, would gladly announce that their nerves are frazzled by the Nicaraguans and want only U.S. assistance to throw them out.

The next item in the invasion kit is the selling of the "danger" to U.S. citizens in the targeted country. In Nicaragua, no problem. If the some 5,000 Americans living there are not endangered now, they certainly would be by the time word got around that Reagan was about to strike again.

In other words, if there is not a hostage crisis, create one.

The American medical students who were rescued by our Rangers are persuaded that their lives were at risk -- as they may well have been once the fighting started -- and the chancellor of St. George University where they have been studying, who had been insisting that he had made less drastic, but still satisfactory plans for their departure changed his mind after he was flown in here for a long session at the State Department.

Anyway, questions raised by sissies who stubbornly wonder if there might not have been another way, can be answered by a few photographs of grateful Americans kissing the ground when Managua is liberated.

Additional, retroactive rationalizations can always be found once things get under way. U.S. troops in Grenada discovered huge caches of Cuban arms. They would find no end of weaponry in Nicaragua, since they have been stocking up since Reagan began to snarl at them. It's another thing he holds against them. How dare they accept Red arms just because they have been threatened by the world's greatest military power?

Another lesson learned by the White House warriors: Exclude the press from witnessing the messy part of "restoring democracy" at gunpoint. Grenada was off limits to all foreign reporters. When several, including The Washington Post's Edward Cody, got there on their own hook in a fishing boat, they were captured by U.S. forces and held incommunicado in "preventive detention" -- for their own good, the Pentagon explained.

It was hardly democratic, and not perhaps the best first lesson to teach the Grenadians, who are said to be attracted to the Soviet approach, but Reagan knew that Americans would only be upset by the sight of their boys shooting and being shot at by people they didn't know they had a grievance against. They are distressed enough by the news from Beirut, and hearing about more carnage could give war -- even one you're bound to win -- a bad name.

Reagan learned from Grenada that he doesn't need to worry about the opposition. The Democratic House Speaker Thomas P. O'Neill Jr. is his staunchest ally, and front- running candidate, Walter Mondale, ever cautious, conveniently did not say that the invasion was, on its face, wrong. He wanted to hear more about the "justification." He would perhaps wait until he found out if the Nicaraguan invasion was "working" before commenting.

Only in Europe were the reviews of Grenada all thumbs down. Margaret Thatcher, of all people, didn't care for it. She has been trying to tell the Brits they can rest easy with Reagan's finger on the button of the missiles that are about to be deployed. It's not so easy now.

George Shultz hurried over to assure her, and the other disapproving NATO allies, that no matter how he seems to be trampling on the rule of law, the American commander in chief is really not John Wayne. Shultz may get the hang of it on this trip. He may need it for Nicaragua.