THE SUCCESSIVE votes in the House and Senate to release MX funds constitute a major political success for President Reagan. He had invested immense prestige in the outcome, and another failure would have been devastating. Three things made the difference. He lobbied hard. He changed his own approach (setting up an outside commission) and his own substantive position. And he drew on a spreading feeling that the country had excessively politicized a critical aspect of defense. It was fitting that he should come to appreciate the virtues of consensus in this field, since no one had done more than he in the late 1970s to block it.

In his op-ed article on Tuesday, Mr. Reagan gave arms reductions pride of place over his two other MX goals--modernizing forces and maintaining deterrence. Certainly in terms of his requirements in Congress, the emphasis was well chosen. The swing votes for MX were supplied by legislators who accepted with the Scowcroft commission that the military case for the MX was not very good. They would not have been inclined to vote for the missile merely to bail the president out of a political fix. They could accept it, however, as an incentive to the Soviet Union 1) to negotiate at Geneva and 2) to move toward less vulnerable and less threatening small, single-warhead missiles in the future.

What Congress approved, then, was not simply a missile. It approved the Scowcroft package including the MX and Midgetman and a new address to the START talks on limiting intercontinental arms. In this light, the ball is now back in the president's court. He must make good on his promises, made in his letters soliciting congressional support for the MX, to review his position at START.

Some confusion arises from the fact that Mr. Reagan's pledges on Midgetman and on "build- down"--dismantling old missiles as new ones are deployed--produce different arms-reduction numbers. This will have to be straightened out. The more serious immediate difficulty, however, arises from Pentagon statements that the Scowcroft package requires "nothing new" from the United States at START.

Congress is focused on strategic issues now. The swing legislators are of a mind to expect that all sides adhere to the Scowcroft bargain. On June 8 the START talks are to resume in Geneva. It would be appropriate, and consistent with his earlier practice, for Mr. Reagan to announce by then not the details, of course, but surely the broad outlines of his new negotiating instructions. As early as June 9 another set of MX money votes is due to come up in Congress.