Former defense secretary Donald H. Rumsfeld is the leading candidate to succeed Robert C. McFarlane as President Reagan's special Mideast envoy, but Reagan has not made a formal decision on the appointment, senior administration officials said yesterday.

According to the officials, the choice of Rumsfeld to take over the job of trying to negotiate a settlement of the Lebanese civil war has been urged strongly on the president by Secretary of State George P. Shultz and White House chief of staff James A. Baker III.

During deliberations two weeks ago that resulted in McFarlane's appointment to succeed William P. Clark as Reagan's national security affairs adviser, Shultz is understood to have proposed that the security post go to Rumsfeld or Brent Scowcroft, who held the job under President Ford.

In his nationally televised speech on Lebanon and Grenada Thursday night, Reagan said he wants to name a new special Mideast envoy quickly to help further negotiations scheduled to begin Monday in Geneva between Lebanese President Amin Gemayel's predominantly Christian government and its Moslem and Druze opponents.

The officials said the delay in the Rumsfeld decision appears to be related to lingering political questions about his acceptability to Republican Party conservatives and to whether Rumsfeld can easily give up his position as president of G.D. Searle & Co., a Chicago-based pharmaceutical manufacturer.

However, the officials said that these problems apparently are being resolved and that the announcement of Rumsfeld's appointment could come early next week.

The main burden of the Lebanon negotiating effort is being borne by Richard Fairbanks, Shultz's special Mideast representative. But Fairbanks, who had been working with McFarlane, is understood to have expressed a desire to return to private law practice, and it is not clear whether he will keep the job after a new presidential envoy is chosen.

Rumsfeld, 51, served as defense secretary from 1975 to 1977, after amassing a long list of credits in various high-level government posts during the Nixon and Ford administrations.

Formerly a Republican member of Congress from Illinois, he joined the Nixon administration in 1969 as director of the Office of Economic Opportunity and then became director of the president's Cost of Living Council.

In the Ford administration, he was ambassador to NATO and later White House chief of staff. When Ford fired James R. Schlesinger as defense secretary during a 1975 shakeup of his national security apparatus, he appointed Rumsfeld to the Pentagon post.

During that period, Rumsfeld formed close associations with Shultz, who held three Cabinet-level jobs under Nixon, and with Baker, who was a key Ford aide. But many influential figures on the Republican right have regarded him as too moderate, and when Reagan became president in 1981 it quickly was made known that Rumsfeld was not being considered for major jobs.

However, the officials said, his supporters for the Mideast post have advanced several cogent arguments in his favor: Rumsfeld, they said, is well versed in foreign policy matters, has "big-name visibility" among foreign leaders and is known as a tough, skilled negotiator.

In addition, the officials said, his reputation as a moderate is likely to be helpful as Reagan seeks bipartisan support to stave off new pressures for pulling U.S. Marines out of Lebanon in the wake of last Sunday's Marine headquarters bombing with its devastating death toll.