In the rush to find simple phrases so loved by the media, instant analysts of the press have coined another one. Ronald Reagan, they are saying in the aftermath of his great tax victory, is "King of Congress."

So he is, for the moment. The Democrats, if not exactly his loyal subjects, are his serfs. They are his to command.

The president has his way on all he wishes. He shatters the ranks of his opponents, and leaves them in the position of political supplicants: be kind and merciful, O Mighty King, and we will serve you.Recalling Virgil, they can implore him:

"Thou, Roman, be mindful to rule the peoples with imperial sway (these shall be thy arts), to impose the way of peace, to spare the conquered and put down the proud."

In the glow of such great triumphs, the Reagan people are entitled to bask in their success. They have demonstrated the power that personality still can play in American politics, and how effective a smooth, tightly controlled and defined political operation still can be, even when governing grows more difficult.

Whether that means a coronation has occurred is something else. For if it has, the president would be wise to remember that old adage, "uneasy lies the head that wears a crown."

His could prove to be of the shaky kind. At the least we'll now know for certain, but not for soon, answers to the principal riddles surrounding Ronald Reagan. That is, whether his political approach holds more promise than reality or, in fact, signals the beginning of a long reign in a new era of American life.

As for the Democrats, their pulse surely beats weakly but, as Mark Twain said in a more personal context, reports of their death are premature and probably totally incorrect.

Early last spring, when prospects for Reagan's success with Congress remained highly doubtful, one of Washington's most astute political observers made a private prediction. The president would get virtually everything he wanted from Congress this year, he believed.

This person, a leading Democrat with long and critical decision-making experience in a number of administrations, offered an analysis that remains provocative months later. In his view Reagan's greatest political danger lies in a cast of mind. What he said stems from a great conservative myth in the country, that "if you're rich you're smart."

He went on to say:

"The far-right conservative mind is to me an utter marvel. I encounter it from time to time and I am fascinated with it. I watch it and I look at it as I might look at something that moves under a magnifying glass. I have a number of friends, officials of companies that I've had relationships with for 30 years, who are extraodinarily conservative individuals. And I sit and watch them in wonderment.

"The wonderment is how individuals can so lose touch with reality. There's a lot of that now. Now Mr. Reagan is a remarkably charming fellow, and I respond to the instincts he has as to how he should react in public situations. As a producer, I sit back and watch and say, 'Splendid performance.' But Mr. Reagan has grown up out in California with a group of conservatives about as conservative as you can get. He grows up with that group and spends a lot of time with them and they finance him and he begins to soak up that stuff. And out of that flows the domestic economic program on which he has staked his future.

"I have the feeling he's going to get his program, 97, 98, 99 percent of it. And as far as I'm concerned that's all right. Now it's all right for two reasons. First place is, I don't know. I'm not an economist and it might work.And it it might work, it would be glorious. If somebody comes along with a whole new theory and it works and our factories and businesses gear up and out percentage of unemployment goes down and our dollar strengthens and our taxes go down, why that's the dream of the ages to have all that come about. If there's a chance for that to come about, why I'd sure like to give it the chance.

"Now do you know what? The chance of it working, in my opinion, is extremely small. It's minute.

"We've gone along with a certain theory of responsibility that the government has for the people of our country, and now comes along a group with an entirely new theory. And it I am to understand the theory, I have to oversimplify it. That's a little better for me anyway, because as I listen to economists I have a tendency to have a curtain come down over my consciousness. But common sense, or what seems to be common sense, oftentimes is a very useful substitute for the science of economics, and common sense tells me something's wrong about this new theory in which the basic factors are:

"We must balance the budget. That's terribly important. Now the first way we're going to go about balancing the budget is cut a number of expenditures. Those are the ones many people depend on, but the country's filled up with deadbeats and crooks and a alot of that kind we oughtn't be looking after anyway. They're leeches on the body politic. So the hell with them, you see. That isn't the way this country became great.

"Now the next thing we do after trimming all that money out, maybe 30 or 40 billion dollars worth, is we're going to cut taxes. We're so confident of the scientific validity of this theory that we're not just going to cut taxes, we're going to cut 'em for three years. Because as scientists if your theorems are correct then based on a sullogistic form of reason we will say that if you go from A to B, B will lead you to C, C will lead you to D and D will lead you to E. And E is our goal.

"Now another thing we have to do as part of this whole plan is increase enormously our defense expenditures. They're already pretty high, but we're going to spend tens of billions of dollars more. So if you stick with us, what this is going to do is cause an outpouring of expenditures and money saved in taxes, and that means people are going to be spending a lot moe and factories are going to be humming and the rate of production is going to go up and everything is going to be in Beulahland. The rest of you benighted people who have struggled along under this basically fallacious system are going to marvel at the results of what we achieve.

"I am convinced in my mind it is not going to succeed. I don't believe it contains the elements that from my experience persuade me it's going to be the success they say it will be. Now the people believe what they hear. The statements have been quite extreme. The sense of enthusiasm is infectious, and I think it leads to hyperbole: this is what you will receive if you go with us. You've just got to have faith.

"I don't believe it's going to happen. I think it's been oversold. I believe they will not be able to produce. I don't know that two years will demonstrate that. The mathematics of 1982 I read as against the Democrats, and it could well be there'll be al kinds of explanations between now and then about why it hasn't worked yet.

"But what I'm suggesting to you is that by 1984 the fallacies of this economic theory will be glaringly and dramatically evident. That's my theory. And as this policy begins to show the defects I think are inherent in it you will begin to see the Democrats rise in proportion as the program begins to show strains."

We shall see. The theories are about to collide. Reagan has his firmly in place, and make no mistake about what it represents: it stems from the rich and powerful, and is heavily weighted in their behalf. Whether it works for the rest of the people forms the great test that now faces the president and his program.