AN AUTHENTICALLY national policy on strategic arms may be taking shape. This rare prospect follows from Pesident Reagan's decision to reverse course and to try to take the politics out of key nuclear questions by submitting them to the consensus-seeking ways of the Scowcroft Commission.

Many in Congress feared that Mr. Reagan would pick and choose among the commission's recommendations and use them simply to renew his earlier single- minded pursuit of the controversial MX missile. But in recent days he has gone far to show he is accepting the commission's recommendations in the compromise spirit in which they were offered. As a result Congress is starting to swallow what it considers the least palatable part of the Scowcroft package, the MX.

Mr. Reagan now agrees to review his position at the START talks with the Russians in order to make it conform with the commission's emphasis on shifting the land-based part of the American nuclear deterrent over time from big multi-warhead missiles to small single-warhead missiles designed to be more secure and less threatening. To this end, he promises to put the single-warhead missile program into high gear, as the Scowcroft panel urged.

In addition Mr. Reagan is showing himself open- minded at the least to two other promising ideas arising in Congress. The first is to propose to Moscow a negotiated agreement on a "build-down"-- retiring two old nuclear weapons for each new one deployed. Some such formula linking arms control and force modernization has high political and substantive appeal.

The second congressional idea is to set up what is in effect a permanent Scowcroft-type public commission "to provide advice and continuity" on strategic issues, as Mr. Reagan put it last Wednesday in carefullly granting "merit" to the idea. The success of the Scowcroft commission, which went out of business with its report, has given powerful impetus to the idea of making such a body a regular part of the Washington furniture.

Mr. Reagan's immediate purpose in making these gestures to Congress and his critics is plain: to win support for the MX, a weapon he continues to regard as vital for both security and bargaining reasons. Even here, however, he has edged back a bit in evident response to congressional and public concern. The figure of 100 MXs is no longer front and center in the president's statements; now he would adjust the level of MX deployment to "Soviet strategic programs and arms reductions agreements." With conciliatory moves like these, it would have been surprising, and disappointing, if key committees in both houses were not going along with him toward the new MX package.