The Senate refused for a third time yesterday to shut off its nearly month-long filibuster over the administration's labor law bill, with both sides conceding that today's expected showdown could hinge on a single vote.

The bill's supporters picked up five more senators in yesterday's 54-to-43 cloture vote. The previous cloture vote, held last Thursday, was 49 to 41.

The third attempt went about as expected, but nose-counters said that as of late yesterday they were still one vote shy of the 60 votes necessary to the filibuster.

"We're close but not over the top," said an administration source, who added that he expected to have 60 votes, possibly more, by today.

Labor sources indicated that their "hard count" for today was 59, and Sen. Orrin G. Hatch (R-Utah), leader of the filibuster forces, acknowledged that the bill's proponents had at least 58 votes and possibly 59.

Yesterday's gain resulted largely from picking up votes of senators who did not vote Thursday. Only Sen. Robert T. Stafford (R-Vt.) switched.

Sen. Lowell PL Weicker Jr. (R-Conn.) was absent yesterday, but has voted for cloture in the past. Some of the bill's supporters also said they expect to pick up the votes of Sen. Charles H. Percy (R-Ill.), Ted Stevens (R-Alaska), John H. Heinz III (R-Pa.) and John Sparkman (D-Ala.) today, with several others possible.

In a carrot-and-stick maneuver, Senate Majority Leader Robert C. Byrd (D-W. Va.) warned his colleagues yesterday they may have to return after the November elections if the filibuster doesn't end soon. But he also said he would try to get the House to approve any bill that the Senate finally passes.

This would avoid the need for a Senate-House conference to iron out differences, which the bill's foes have charged would lead to conferees' acceptance of a tougher bill already approved by the House because most of the likely Senate conferees are supporters of the bill. It would also avoid another opportunity for a filibuster.

There is nothing essential about getting cloture today, but the vote has been billed by both sides as the crucial test. Hatch and others contend that the proponents may fold if today's vote fails. But Byrd, in an apparent signal that this won't happen, filed another cloture petition for tomorrow.

However, sources said the possibility of absentees tomorrow and Friday means a fifth cloture vote - if necessary - might not occur until next week.

Earlier in the day, Senate Minority Leader Howard H. Baker Jr. (R-Tenn.) said he was confident of holding on to 41 votes at least through today's vote and predicted, "If we survive this week, they (the Democrats) may take it down," meaning drop it from the Senate's calendar.

The bill, backed by organized labor over heavy opposition from business groups, would set deadliness for union representation elections and stiffen penalties against employers who violate labor laws to thwart union organizing and bargaining drives, including time-and-a-half back pay for illegally fired workers and cancellations of federal contracts. Byrd last week offered several proposed modifications, but opponents have labeled them unacceptable.