IN THE MILLING over the deficit, the president has now turned, as threatened, to the veto. Last Friday he returned to Congress, with a message of disapproval, the Treasury-Post Office appropriations bill. The bill is $180 million over the budget resolution that Congress adopted and the administration accepted in August. It would take a thousand such vetoes to balance the budget.

Mr. Reagan continues to think that Congress and domestic spending are the problem. Congress with its well-established weaknesses is the perfect foil for the president, a pillow he can punch at will. Thus the president declared last week that the fiscal procedures in Congress have broken down and that Congress has an "ingrained incapacity to tackle the large budget deficit." He said that "the old propensity to spend and spend and to capitulate to one interest group after another continues unabated."

All true, or near enough -- but not an explanation for the deficit. Congress has been weak a lot longer than the deficit has been this large. When he came to office in 1981, the president adopted a guns-and- butter policy by another name, cutting taxes while advocating a defense buildup and offering the assurance that either economic growth or domestic spending cuts would take care of the deficit. They haven't, and Congress has urged the president to consider either a tax increase or defense stretch- out instead. He won't.

The president observed that in his budget last February he proposed cuts or termination of about 50 domestic spending programs. He said that "Congress has accepted very few of these proposals, and every nondefense appropriations bill will far exceed my budget." But these rejected proposals -- some of which are good ideas -- are not themselves enough to take the deficit down to an acceptable zone. And they have been rejected; to some extent that means they are no longer practical proposals.

The Treasury-Post Office bill may have deserved to be vetoed. The House leadership has conceded it does not have the votes to override. But Congress, in its work so far on the appropriations and other bills to carry out the budget resolution, has pretty well abided by that measure. Vetoes are fine for scoring points, but they are not an answer to the deficit.