No wonder he beams and thrusts out his jaw and poses for the cameras by clasping his hands above his head in the universal signal of The Champ. President Reagan, for the moment, stands as political master of all he surveys.

Not since Lyndon B. Johnson dispatched his paper blizzard of legislative Great Society proposals that were swiftly enacted has a president so dominated the Congress as has Reagan in recent days.

Four times, twice in each chamber, members of the House and the Senate were put to a test on an issue in which the president staked his political capital and personal prestige. The issue, of course, was whether to release funds for building more MX missiles -- about which, to put it mildly, lawmakers of all political stripes held grave doubts for military, strategic and economic reasons.

What's more, public sentiment about huge outlays for defense at the expense of domestic spending clearly has been turning against the Pentagon in recent weeks. As the time for the votes neared, page one headlines and network newscasts would seem to have bolstered antidefense spending feelings.

Further examples of Defense Department waste were being reported daily. Huge Pentagon contractors were being grilled in hostile Capitol Hill hearings about their cost overruns and their soaring profits. Finally, a federal grand jury indicted General Electric, the fourth-largest military contractor, on charges of falsifying $800,000 in defense business claims and of lying to the government about its work on a nuclear-warhead system.

Result: victory, total, for the president. Virtually no breaks in the ranks against him, despite all the intensive lobbying and all the critical publicity. Few new dissenters, despite a presidential argument that stood logic and history on its head. However the lawmakers may have felt privately, they bought the president's line.

In the aftermath of the decisive votes granting congressional approval for the missiles, it's that presidential line -- or rationale, if you want a kinder word -- that deserves a cold, hard, sober look now.

Members of Congress bought Reagan's argument that they should vote for MX as a "bargaining chip" to strengthen America's hand during arms control talks with the Soviets in Geneva. Failure to vote for the MX, the administration reasoning went, would send a signal to the Soviets of an irresolute, weak America and encourage them in the belief that they could proceed at will with their arms buildup.

In fact, as former senator J.W. Fulbright (D-Ark.) observed in a conversation we had after the final MX vote last week, just the opposite signal might have been sent to the Soviets: that the United States isn't really serious about disarmament and that this administration is determined to go ahead with the vastly greater expenditure of funds to build the "Star Wars" defense system. Thus, instead of a move that would help the cause of disarmament, the latest MX vote could lead to an even greater acceleration of the destructive arms race.

This MX "bargaining chip" argument, as Fulbright and others pointed out during the debate, was the same one the Reagan administration used two years ago when it asked Congress to approve $2.1 billion for the purchase of the first 21 MX missiles. Unless Congress voted then to authorize and appropriate funds for the missiles, the administration warned, arms talks with the Soviets would be endangered. Congress bought that line. What happened? Just one month later the Soviets broke off the arms control talks because of the administration's decision to deploy Pershing and cruise missiles in Europe.

To Fulbright, that Soviet response was predictable. History teaches that they will match any buildup of ours. Correspondingly, only when there's a real sign of an American move to reduce the arms race have they responded positively.

The great example of that came after President John F. Kennedy's American University speech in June 1963. Kennedy took the initiative in calling for a reexamination of attitudes about the Cold War and made a dramatic, specific proposal to seek a treaty to outlaw nuclear tests, then being carried out extensively in the Earth's atmosphere. Out of that came an instant and favorable Soviet reaction, the initiation of serious talks and the signing that August of the Limited Test Ban Treaty, a small but significant step toward slowing the arms race by eliminating atmospheric testing.

For once, I hope a Fulbright analogy is wrong, or, in this instance, inapplicable. But I fear that, as usual, his view will prove to be sound. If so, the congressional vote on MX will not have made America more secure. It will have made the world more dangerous.

NOTE: In last Sunday's column I explained the Gridiron Club's name by saying it stemmed from "the cooking implement, not the field for football maneuvers, since the game of football was unknown when this group came into being" 100 years ago. This turns out not to be true, or, more accurately, only partly correct. The club took its name from the cooking implement all right, and the term "gridiron" was not yet associated with football. That now-common usage first saw public light, according to the "Dictionary of Americanisms" (University of Chicago Press) on Sept. 5, 1897, when a sportswriter for Hearst's New York Journal told readers that a football captain "will marshal a small army of gridiron warriors on the varsity athletic field." But the game of football predates the newspaper exercise in satirically "grilling" politicians on the gridiron each spring in Washington.

As Prof. Emeritus Edwin L. Stevens of George Washington University writes: "In 1869, 15 or so years before 'The Club' was founded, Rutgers University and Princeton University played the first intercollegiate football game in New Brunswick, N.J. Rutgers won, but didn't beat Princeton again for 69 years! Quite a losing streak. Thus football can't properly be said to have been unknown when the Gridiron Club was founded."

I'm pleased to correct the record, with a bow to Prof. Stevens, who, you might guess, is a Rutgers graduate, Class of '36.