Quite apart from the national security gamble involved, Ronald Reagan is playing high-risk politics--internationally and domestically--with his "bargaining chip" approach to arms control. The risk is obvious in the blunt warning welling up from some Republicans, as well as Democrats, who swallowed hardest before voting to go forward with the president's MX missile program.

Having given the president one of the chips he insisted was essential (under the scarcely veiled threat of being hit with a "Who lost arms control?" charge), they are laying claim to a clear covenant and hastening to nail down his end of the deal: significant modification in the U.S. position on strategic arms reduction talks in Geneva.

If the president cannot demonstrate a good-faith effort and some promise in his arm-to-parley approach, the Democrats may well have the best of the argument in the inevitable arms control debate in next year's election.

Less obvious but more far-reaching in its repercussions is the risk in the president's new line on negotiations to control medium-range nuclear weapons. Here the bargaining chip is the 1979 NATO decision to begin deployment of Pershing II and/or cruise missiles in Western Europe by the end of this year to match the threat of Soviet SS20s.

The new Reagan line, conveyed in a stage-setting, pre-Williamsburg interview with foreign journalists, is that only the actual installation of the first American missiles will concentrate the minds of Soviet negotiators. "Once they see that we and our allies are determined to go forward with the deployment of these weapons, then I think they might meet us in legitimate negotiations," the president said.

A good many specialists in and out of government believe he may be at least half right. That is to say, they agree that the Soviets may well string out the negotiations right up to the deployment deadline. For all their stalwart talk, the European leaders in one or another of the key countries involved may yet crack under the pressure from their "peace" movements as deployment day gets closer.

West Germany is the Soviets' prime candidate. The key to West Germany, once again, will be its public's perception of American good faith in its reach for compromise. Italy looks relatively secure. And so, right now, does Britain.

But not even Prime Minister Margaret Thatcher is taken for granted, despite her apparent commanding lead in next week's British election. Anything short of a clear- cut Tory victory would play into the hands of Britain's formidable anti-nuclear forces. British second thoughts, in turn, could have a "ripple effect" on the continent, a State Department expert warned.

But let's say Reagan is right and this game of chicken plays out; the Russians hang tough and the U.S. missiles move in. Does it follow that the Soviets buckle? Is it not more likely that they mean what they say, which is, in effect, that two can play with bargaining chips?

Most of the same experts who say the Soviets won't get serious until U.S. deployment starts are equally persuaded that the Soviets do mean what they say-- that one way or another they will counter any heavying-up of nuclear weaponry on the European side by some compensating heavying-up on their side.

The most far-out scenario one hears would be Soviet missile installations in a congenial Central American country-- Cuba or Nicaragua. Almost as unlikely would be a first Soviet deployment of medium-range nuclear weapons in a Warsaw Pact country; the most probable choice, for its effect on West Germany, would be East Germany.

The speculation runs a wild and woolly gambit. The simplest, safest Soviet response would be to add more SS20s in the Soviet Union, targeted on Europe. But the experts do not exclude deployment of comparable weapons in the corner of the Soviet Union closest to Alaska, capable of hitting not only that piece of U.S. territory but parts of Canada. It is reliably reported that over at the Pentagon, where people are paid to contemplate "worst cases," the war gaming envisages U.S. missiles deployed in Alaska against Soviet territory as a logical counter-measure.

There is nothing to say that at a fairly early stage the game of bargaining chips might not have the desired effect: serious bargaining. But it might also become hard to avoid at least the impression of an arms race out of control. It is hardly necessary to spell out the potential implications that could have for already precarious allied relations, for the U.S. arms-control debate-and, at bottom, for U.S. security.