A FORTNIGHT after President Reagan's MX decision, it is plain that the proposal is in for a long, hard and uncertain run. It passed its first hurdle in the House Appropriations Committee only by one vote, on a tie, and the committee hung on an amendment extending the program's window of political vulnerability to March 15, by which time a new House with 26 additional Democrats will have been seated. The proposal is not obviously doomed. Some 10 of 31 Democrats on Appropriations were ready to start building the missile. But the struggle may be a cliffhanger all the way.

So far, the MX fight is the most serious discussion of its sort since the aborted SALT II debate of 1979. In many ways, it is the extension of that debate, and desirable precisely for that reason. Such events compel politicians to draw together the disparate elements of national policy on the great and elusive questions of war and peace -- considerations otherwise left to special constituencies and bureaucracies. How much defense do we need? To meet what threat? At what cost? Accompanied by what relationship with the Soviet Union?

The care -- the hesitation -- with which Congress is closing on the MX is itself a sign that an earlier consensus, the one that helped bring Mr. Reagan to office, has eroded. He arrived at the White House, after all, fairly claiming a mandate to build up our arms. But people now have a sense that too many of the separate hardware, budgetary, doctrinal and political choices that the president has made in the name of his mandate have been arbitrary and ill-considered. He has yet to project a sense of a purposeful design of which these choices are logical and vital parts.

Nowhere is this lack clearer than on the question of whether the country should still be planning to rely to the extent that it does on land-based missiles that seem destined to remain vulnerable, no matter what, to a Soviet strike. This has seemed to us the threshold question, and it is treated on the opposite page today by Christoph Bertram, a European specialist. The administration is more interested in addressing the Popular Mechanics question of whether the MX will work and the bargaining chip question of how building or not building it will affect arms control talks with Moscow. These are not insubstantial matters, but first, we believe, it must be determined whether, given the relentless advance of attack technology, land-based missiles ought to retain their familiar place in American deterrence. We think not.