President Reagan showed extraordinary confidence in his command of two political systems, the American and the Soviet, in declaring that the restraints of the SALT treaty would no longer bind his decisions on strategic offensive forces. He is taking extraordinary chances on both counts.

He is gambling that in formally letting go of SALT he will still be able to keep Congress and the public behind the arms-building plans that, he insists, are essential not only to ensure American security but also to bring the Russians to new and improved arms control terms in Geneva.

Here he is toying with a central political truth stated by Richard Nixon. It is that it's hard to sustain popular support for defense spending if the public isn't convinced the administration in power is working hard for peace. Either people get dispirited, suggested the former president, or the opposition starts fighting the administration's foreign policy by taking its defense requests hostage.

It was apparent already that in a Gramm-Rudman budget year, with defense management under heavy attack and with specific programs such as the president's Strategic Defense Initiative under especially heavy attack, Reagan was going to meet trouble on the defense side. This Congress, after all, has already locked up his program for testing anti-satellite weapons. It did not take great prescience to calculate that by ditching SALT he would tempt more budget trouble. But he has brought it on himself, rekindling some of the same alarms about his nuclear trustworthiness that flourished in his earlier White House years.

He has stirred further concern about his administration's internal balance. The growls of the far right that Reagan was going pragmatic have actually comforted the center and left. These elements could live with the far right, even while combating it, as long as it was supplying bargaining leverage to the more pragmatic figures in the administration but was not prevailing itself. This time, however, the far right did not simply apply pressure and dutifully lose. It won. Since so much can flow from this decision, prudent people have cause for concern.

My own concern centers on Secretary of State George Shultz, who, with Robert McFarlane gone as national security adviser, carries the full burden of representing reason in high policy councils; McFarlane's successor, Adm. John Poindexter, seems weightless. Mystifyingly, Shultz appointed to the key departmental posts persons unable to help him enough in the critical area of Soviet-American relations and strategic affairs, an area in which he was poorly prepared and in which he still does not speak with full command, to judge by his public remarks.

The practical effect of the deficit of pragmatism inside the administration is to strengthen the impulse of Congress and other outsiders to fill the gap. This is, of course, a formula for further politicizing a sensitive policy realm. It's a troublesome course, but it beats giving the administration's hard-liners a clear field.

And the Russians? The view comes naturally to the Reagan administration that communists are a generic breed -- they're all alike. But it is possible to discern in and between the lines of the statements of Mikhail Gorbachev and his comrades an argument not unlike the one between the Reagan administration's leading figures, whose facade of general like-mindedness masks specific differences under the surface. Broadly speaking, Soviet hard-liners think it is illusory to seek a measure of renewed de'tente, especially with a Ronald Reagan, and Soviet pragmatists, including Gorbachev, think it is necessary.

It is wrong to think of playing to the Soviet pragmatists by reaching out to them or "rewarding" them: that way lie endless one-sided American concessions. But if a deal is reasonable and in the American interest, even if it's not everything, it should be pursued.

The margin in Moscow for dealing with Reagan has got to be greater than the margin in Washington for dealing with Gorbachev. Reagan trifles with it by pulling out of SALT. Reagan may think he's tightening the screws from a position of economic and technological strength. The Russians may think he's demonstrating, wittingly or unwittingly, that he believes he can push them to the wall. Reagan and his kind like to feel they're being tough and hardheaded. By acting as though they can have their way with the Russians without compromising, they're being romantic and softheaded.