As the trade bill moves to the Senate for vote on the conference committee report, the future of the bill rests with a handful of undecided senators.

The vote was scheduled for Tuesday, but Senate majority leader Robert Byrd, D-W.Va., said a vote could come as early as today.There is widespread agreement that the votes are there to pass the bill, what is less certain is whether support can be mustered to override the veto President Reagan has promised.

Presently it seems very likely that the bill's supporters will not be able to convince the necessary two-thirds of the senators to vote to support the bill.

Sixteen senators appear to be the key to the Senate vote on the trade bill, and on a possible veto. Eight remain undecided; five oppose the legislation or are leaning toward opposition; three will vote for it.

I don't think they're going to get the 67 even on the first go-round, said one Senate staffer.

The president has vowed to veto the bill if it comes to him in current form. His chief objection is the provision that mandates employers give 60 days' notice before shutting down a facility or laying off 500 or more workers.

The plant closings amendment was part of the Senate bill that passed that house last summer. It was approved by a margin of 60-40, seven votes short of the the two-thirds required to override a veto.

The bill itself passed by 71-27, but a number of Republican senators, led by Sen. John Danforth of Missouri, voted yes but, meaning they voted yes but would not vote the bill into law unless significant changes were made.

One major change these senators sought was the removal of the plant closings amendment. Atotal of 16 senators - four of them Democrats - voted against the plant closings amendment but voted for the trade bill.

Those senators are: Sen. Danforth, Sen. John Chafee, R-R.I., Sen. Strom Thurmond, R-S.C., Sen. John Warner, R-Va., Sen. Frank Murkowski, R-Alaska, Sen. William Roth, R-Del., Sen. David Karnes, R-Neb., Sen. Robert Kasten, R- Wis., Sen. Charles Grassley, R-Iowa, Sen. Don Nickles, R-Okla., Sen. Larry Pressler, R-S.D., Sen. Paul Trible, R-Va., and the four Democrats - Sen. Ernest Hollings, S.C., Sens. Dale Bumpers and David Pryor of Arkansas and Sen. David Boren, Okla.

Among the four Democrats, Sens. Bumpers, Pryor and Boren say they will vote for the conference report. The two Arkansas senators will support the bill because it provides $5 million in export assistance grants to small businesses as well as increasing the lending limit for small business loans.

Sen. Boren will support the bill because it has much to offer the oil industry, especially repeal of the windfall profits tax.

Sen. Hollings remains undecided, with plant closings his primary objection. However, Sen. Hollings did sign the conference report, as did all other Democrat conferees except Sen. William Proxmire of Wisconsin.

On the Republican side, Sens. Trible, Pressler and Murkowski will vote against the bill if it contains the early notification measure.

An aide to Sen. Murkowski hinted that the senator may filibuster in an effort to strip the plant closings amendment from the bill.

It's conceivable the senator will have to discuss the trade bill in order to educate his colleagues . . . He intends to vote against the bill and plans to urge the president to veto it, the staffer said.

An aide to Sen. Trible said the Virginia Republican would probably vote against the bill and probably vote to sustain a veto.

Sen. Pressler has signed a letter to the president that states he will vote against the bill unless changes are made.

Sens. Thurmond and Grassley are leaning against the bill because of the plant closings measure and because of several other non-trade measures in the bill including the establishment of a council on competitiveness .

An aide in Sen. Grassley's office said the senator was concerned about the budgetary impact of such measures.

But aides to both lawmakers said the two senators wanted to support a trade bill. Sen. Grassley would like very much to see the agricultural portions of the bill become law, said one aide.

Seven of the senators remain undecided. But of those, Sens. Danforth and Roth signed the conference report. Obviously, this does not mean they will vote for the bill or vote to override any forthcoming veto, although Sen. Danforth is leaning in that direction. It does, however, give the bill's proponents reason for optimism.

Sen. Nickles is undecided as well, but a staffer in his office said the Oklahoma Republican, like his colleague Sen. Boren, is very pleased with the provision repealing the windfall profits tax for oil producers.

Sens. Warner, Kasten and Karnes also would like to see a trade bill. Sens. Kasten and Karnes like the agriculture section of the bill and Sen. Warner likes the improved protection for U.S. intellectual property rights and the liberalization of export controls.

Sen. Warner's aide said, however, that he does not like the transfer of some authority away from the White House to the office of the U.S. Trade Representative.

All three men remain strongly opposed to the plant closings provision.

Clearly, this poll of 16 senators is not conclusive. Any number of senators who voted to support the bill last summer could change their minds. But vote counters in the Senate say these eight undecided senators are key to the bill's future.

Assuming the five Republicans who oppose or seem to be ready to oppose the legislation stick to their guns, the administration needs only to sway two or three votes to have the president's veto sustained.

At this stage that looks like a pretty good bet.

If that's all they have to do, said one Senate staffer, I'd say they are going to do it.