President Carter held lengthy meetings with senior advisers yesterday on a possible veto of the $36 billion weapons procurement bill, and has called congressional leaders to the White House today to discuss the legislation.

Some adminstration officials close to the situation said they believe Carter has tentatively decided to reject the military bill, which would be an action with little or no precedent in recent times. But other officials said they had been told after a presidential meeting, which included Secretary of Defense Harold Brown and senior White House and Budget Bureau figures, that no final decision has been made.

Carter has until midnight tonight to either sign the bill, veto it or allow it to become law without his signature. He is expected to announce what he will do in a televised news conference scheduled for 4 p.m. today. It will be carried live on channels 4, 7 and 9 and on tape at 11:30 p.m. on channel 26.

The main item of dispute is a giant, nuclear powered aircraft carrier, costing $2 billion, which was added by Congress after an active lobbying campaign by nuclear seapower advocates, including some uniformed officers of the Navy.

A veto would pit Carter, a former nuclear officer of the Navy, against the strong sentiment of many in his former service, including his naval mentor, Adm. Hyman G. Rickover.

There is "very strong sentiment" in the White House for rejecting the bill, an informed official said last night, but also "great concern about the political costs." Congressional sources said White House aides have been making soundings on Capitol Hill in an effort to determine whether a veto of the bill could be sustained.

The administration has been carrying on a lobbying effort to persuade the Senate Appropriations Committee to kill the nuclear carrier by dropping it from the military appropriations bill, which is a companion measure to the authorization bill now on the president's desk. Some sources in Congress, where speculation about Carter's action was rampant yesterday, suggested that the veto talk might be part of the lobbying effort with the Senate committee.

A top defense official, speaking to reporters early yesterday, accused Congress of lowering combat readiness by cutting important parts of the defense budget to pay for the $2 billion carrier. The official, who did not permit use of his name, said the "glamorous" carrier would be at the expense of funds needed to maintain the fighting fitness of warships, planes and other military equipment already in use.

"We have a serious problem of degrading our military capability below what it would otherwise be in order to procure some particular weapon system that seems attractive to some in Congress, the military and contractors. I just don't think we can afford that in the long run," the official said.

The official added that Congress' plan to purchase the nuclear carrier would cut deeply into allotments proposed for strengthening U.S. forces committed to the North Atlantic Alliance and for proposed research and development.

As passed by Congress in its final form, the authorization bill for weapons procurement, military research and civil defense totaled $36.9 billion, about $1.5 billion more than had been recommended by Carter in his budget. The remarks of the top Defense Department official suggested that Carter's attack on the measure, if he makes it, is likely to be centered on the changed priorities among weapons and other defense expenditures rather than on the issue of additional cost.

After the bil was passed, the administration indicated it would accept the authorization of a conventionally powered aircraft carrier, which would cost about $1.5 billion. But Defense Secretary Brown and others remained strongly opposed to the big nuclear ship.