THE ADMINISTRATION has lobbed a grenade into the middle of the already difficult effort to impose discipline on the federal budget. The president confirmed at his press conference last night that he does not feel bound by the compromise on defense spending that was an essential element in congressional agreement on next year's budget resolution. This not only complicates negotiations currently going on in both houses; it also reopens the question of whether Republican congressional leaders can count on the president for any help in their struggle to narrow the deficit.

So far, the president has provided little but verbal support. He sent Congress a budget last January that was so unpopular with all sides--conservatives shied away from its record deficit, liberals decried its harsh cutbacks in social programs--that his own party wouldn't bring it to the floor. He hung back from subsequent budget negotiations until the Senate leadership had delivered a workable compromise, and he stayed out of the Finance Committee's fight for tax reform until the last possible moment.

Meanwhile, Mr. Reagan continues to thump the tub for the pure hokum of a constitutional amendment purporting to require a balanced budget--an obvious embarrassment to thoughtful congressional leaders who see the hypocrisy of supporting the amendment when the president's own demands make record budget deficits inevitable.

To renege on the defense compromise could mean that the deficits in 1984 and 1985 would be $23 billion larger than the already terrifying totals that the Congressional Budget Office predicted earlier this week. The administration apparently sees somewhat lower deficits in its mid-year budget review, but its predictions are still awesome, and there is nothing in the administration's forecasting record that would suggest its numbers should be given the same credence as CBO's.

More immediately, backing away from the defense compromise could imperil progress on the roughly $13 billion in budget cuts for next year that Congress promised to deliver in the budget resolution. It is well known that many members from both parties acquiesced in cutting domestic programs and entitlements only because the agreement also called for cutbacks in the still enormous growth in defense spending. These members may well feel that they are no longer bound by the agreement to cut the domestic budget if the president sees no need to honor it with respect to defense.

The president's decision--apparently made against the advice of all his major advisers except Defense Secretary Weinberger--caught Senate leaders by surprise. Senate Appropriations Committee Chairman Mark Hatfield yesterday expressed the view that higher defense requess would be a "breach of faith." Other Senate leaders are now questioning whether the whole defense budget ought not to be subjected to a thorough congressional review to see what can be saved.

Beneath the concern expressed by Republican leaders over the practical difficulty of accommodating still larger defense budgets runs a disquieting note. Even if the president really feels more defense spending is needed in future years--and is prepared to show how he would finance it--why did he decide to announce it now when they are still fighting to get control over next year's budget? What is the point?