President Reagan joined with Senate Republican leaders yesterday in a major effort to keep bottled up legislation that would impose on South Africa tougher sanctions than those the White House ordered Monday.

In what both sides predict will be a close vote, the Republican-controlled Senate is scheduled to vote today for a second time on cutting off debate on the sanctions bill that is opposed by the White House. A vote to end the debate would be a defeat for the administration, which is attempting to avoid an actual Senate vote and a potential confrontation among Republicans on the sanctions.

Reagan, who announced he would veto the sanctions legislation as he signed an executive order Monday incorporating many of the bill's key provisions, telephoned two or three Republican senators yesterday to urge them to oppose a Democratic-sponsored move to end debate on the issue. An end to the debate would lead to almost certain approval of the sanctions bill and a presidential veto, according to White House officials.

On Monday, the Senate fell seven votes short of the 60 necessary to invoke cloture and force a vote on the sanctions bill.

Although Senate Majority Leader Robert J. Dole (R-Kan.) expressed confidence yesterday that he has the votes to sustain a veto, both he and the White House are seeking to avert a protracted battle over the sanctions legislation that would continue to highlight differences between the administration and some Senate Republicans over policy toward South Africa.

Around Capitol Hill, those involved in the sanctions dispute uniformally predicted a "very close" vote today but suggested that the White House -- given the involvement of the president and Secretary of State George P. Shultz, among others -- will probably prevail.

"Right now I would say that the odds are against invoking cloture by one or two votes," said Sen. Lowell P. Weicker Jr. (R-Conn.), one of 12 Senate Republicans who broke ranks with the White House and party leadership on Monday to support pushing the sanctions bill to a final vote.

Dole yesterday again accused Democrats of turning the sanctions dispute into a partisan issue. For their part, Senate Democrats were savoring the quandary that a number of Republicans found themselves in as they approached a second vote on the sanctions issue amid growing pressure to support the president.

"The Republican Party is at a crossroads on this issue," Sen. Edward M. Kennedy (D-Mass.) said yesterday on the "CBS Morning News."

"It must decide whether to be the party of Lincoln or the party of apartheid," he said, in a reference to the South African government's policy of strict racial separation.

The problem for many Republicans was that a vote in favor of the sanctions bill, which is viewed as stronger than Reagan's executive order, would be seen as a weak response to South Africa's racial policies, while a vote to cut off debate and force passage of the bill would be interpreted as disloyalty to a popular Republican president.

To counter this, according to a Republican Senate staff aide, Dole and Foreign Relations Committee Chairman Richard G. Lugar (R-Ind.) are arguing that Reagan's executive order adopts most of the bill's provisions and that the Senate should "declare victory" and close ranks behind the president.

"This is a vote for a new policy on South Africa and a vote for the president," the aide said.

The White House declined to identify which Senate Republicans were contacted yesterday by Reagan, but attention focused on defectors in Monday's vote and seven Republicans who were absent then. The White House was thought to be concentrating on changing the minds of defectors to offset the expected vote for cloture by one of the absentees, Sen. Arlen Specter (R-Pa.), and possibly one or two others.

According to a source close to the Republican leadership, "several" of the defectors indicated to Dole that they would support Reagan's position if their votes were absolutely necessary.