President Reagan warned Republican congressional leaders yesterday that he intends to veto a $480 billion omnibus appropriations bill if any major category of spending exceeds individual targets that Congress has set for the year.

This means that Congress will face a possible veto showdown on the eve of its scheduled Christmas adjournment if it meets its overall budget target for fiscal 1986 by shuffling money from military outlays to domestic, as it has done in the past.

For those who want to "try to roll excessive spending into one big bill and hope to avoid a veto, I've got news," Reagan reportedly told the GOP leaders.

His news was that he would insist that spending in the omnibus continuing resolution be held to levels set in the congressional budget for each of the 13 regular appropriations bills Congress is supposed to pass every year.

Not only will individual appropriations bills be vetoed if they exceed the targets but "even the continuing resolution would be vetoed -- it will be vetoed," Senate Budget Committee Chairman Pete V. Domenici (R-N.M.) said after the session with Reagan.

The continuing resolution is needed to fund most government agencies, including the Defense Department, because Congress has not yet passed the appropriations to finance them for the fiscal year that started Oct. 1.

It has passed four of the 13 bills, one of which, the treasury-postal appropriations, was vetoed by Reagan.

A previously approved stopgap funding bill expires Dec. 12. The new bill would last until October.

Several categories of domestic spending exceed budget targets in appropriations bills under consideration for inclusion in the continuing resolution in both houses, according to administration officials. They said spending for the Transportation and Interio w0157 ----- r a BC-12/04/85-VETO 1stadd w0157 12-04 A05 people, it's a hard thing to swallow . . . . They the administration keep finding new ways to squeak away from making any cuts in '86," said a Democratic aide.