REPUBLICAN LEADERS in Congress have minced no words in telling the president that any serious assault on the deficit must put a lid on military as well as domestic spending. This is a practical necessity. Add together defense spending -- at almost $300 billion -- and the unavoidable interest costs on the fast-rising federal debt and you account for almost 45 percent of federal spending. Without a massive increase in taxes -- which no one wants -- it is barely possible to narrow the deficit if defense growth is not restrained.

President Reagan and his defense secretary, Caspar Weinberger, appear unmoved by this dilemma. The president seems to believe that the Pentagon -- which, over the past five years, has added $100 billion in inflation-adjusted dollars to its annual budget -- has actually made substantial cuts. But in fact, its only reductions have been those made to correct overinflated budget costs or to meet congressionally imposed delays in production timetables that it might not have been able to meet anyway.

Now the president has apparently signaled that Mr. Weinberger need not take seriously even the modest defense budget restraints -- not cuts, just slightly smaller continued increases -- that OMB Director David Stockman has suggested. Mr. Stockman's plan had called for the Pentagon to trim its next-year increase by $9 billion, and its three-year request by $121 billion. Mr. Weinberger offered $6 billion next year and $19 billion over three years.

That is $19 billion out of Pentagon spending authority estimated at $1.1 trillion over the next three years, and the part of these "savings" that would not come from deflating spending overestimates would come from moving some of a planned military pay increase into the current, rather than the next fiscal year. That way it doesn't count as a spending inease next year even though it would add more to the public debt.

The president is reported to have declined to discuss any details of the defense budget in his two- hour meeting on the subject Wednesday. He told his defense secretary that his token offering was "fine." It looks increasingly as if Congress will have to take on the job of convincing the president that the deficit problem is real.