MX countdown

President Reagan has started the 45-day countdown for Congress to accept or reject his latest MX proposal. Echoing a plan Congress has turned down before, it calls for deployment of 100 10-warhead Peacekeepers in existing Minuteman silos. This time Congress has been candidly advised by the head of an expert presidential panel that the recommendation would probably have been different if politics had not played the part it did in the body's decisions.

Will legislators defer grinding their own political axes for 45 days? Not a very long time to sort out what is militarily necessary from the political context.

Politics, of course, can have a constructive role in ensuring that the wishes of the American people are represented when national security policies are shaped. But both the people and their representatives need the best expert opinion to help them reach sensible conclusions.

Mr. Reagan's Commission on Strategic Forces was looked to as a source of such opinion. Its list of members and senior counselors was bipartisan in a Republican-Democratic sense, including former secretaries of defense and CIA directors from both parties, for example.

But the roster includes no former arms-control agency officials, though the commission cites arms control as a major reason for deploying the MX. SALT I negotiator Gerard Smith has recently prepared a report concluding the MX program is unwise.

Meanwhile, McGeorge Bundy, national security adviser for two Democratic administrations, also finds the MX militarily counterproductive. He writes that the presidential panel was ''carefully selected to include only tested friends of MX.''

Such considerations do not necessarily mean that the commission package, including a single-warhead missile somewhere in the future, cannot be promoted on both military and political grounds. But the administration missed an opportunity to give the public a sense that here were strategic experts providing the bottom line on the weapons the nation needs, letting the chips fall where they may. And then letting the President deal with the politics.

Now Congress has to decide where politics ends and expertise begins. Not all members are equally known for making such distinctions. Considering the stakes during these 45 days, constituents can hardly fail to notice those who do or do not try.