President Reagan's senior staff is expected to undergo another major turnover in the next few months with the departure of at least two top assistants, political director Edward J. Rollins and congressional liaison Max L. Friedersdorf.

Both Rollins and Friedersdorf, veterans of Reagan's first term, returned to the White House at the outset of the second term to bolster the political reach of Donald T. Regan, the former treasury secretary who was then taking over as new chief of staff.

Since then, Regan has consolidated his authority in the West Wing, but officials said his senior staff is still in flux.

Recently, Regan hired a former White House lobbyist, Dennis Thomas, to help cope with a growing workload -- including scheduling and administration -- that the chief of staff had taken upon himself. Now, Regan must also find a new team of political advisers as the president heads into a revived campaign for his tax revision plan this fall and next year's congressional campaigns, in which Republican control of the Senate is at stake.

Rollins, whose advice was sometimes ignored by the chief of staff, has told Regan he plans to leave after Oct. 1, but said he would remain until the November elections (which include governor's races in Virginia and New Jersey) if asked. Rollins is planning to go into the political consulting business.

Regan has talked to former White House political director Lyn Nofziger, now a private consultant, but Nofziger "is making a good living and he doesn't want to come back," according to a senior official.

One possible replacement is Mitchell E. Daniels Jr., currently a deputy to Rollins who is a former executive director of the Senate Republican campaign committee. Friedersdorf is described by colleagues as weary of the White House routine and as less effective this year than he was in helping win Reagan's initial victories in 1981.

Although Friedersdorf is highly respected in some quarters on Capitol Hill, there have been troublesome quarrels between chief of staff Regan and Senate Republican leaders that have marred the White House relationship with Congress this year.

Regan "has to decide what he wants in those positions," said a White House official, referring to the political and legislative posts. The official added that Regan's predecessor, James A. Baker III, now the treasury secretary, "is his own legislative guy. But Don Regan doesn't do that. He doesn't like to sit down and schmooze" with lawmakers.

It is not certain how Regan will replace Friedersdorf, but officials speculate he may turn to M.B. Oglesby, now Friedersdorf's deputy, or perhaps to Thomas, who handled congressional relations at the Treasury Department under Regan.

Friedersdorf has also talked with Regan about leaving in October, officials said, but may remain later. He is planning to return to a diplomatic post he previously held in Bermuda, an arrangement made when he agreed to return to the White House this year.

A third senior assistant whom Regan brought into the White House this year, communications director Patrick J. Buchanan, has yet to fulfill early expectations that he would impose a strong conservative cast on Reagan's second term.

Known as a fiesty conservative columnist, Buchanan discovered once inside the White House that Reagan was not always as ideological as he had thought, officials said.

For example, Buchanan argued for a military response to the hijacking of TWA Flight 847 last month. But Reagan responded by ruling out military force as an option. One official said "it would not be unfair" to say Buchanan "was mildly surprised by that."

Buchanan also has advocated a more confrontational approach to Congress on such matters as aid to the rebels fighting the government of Nicaragua, and has been discouraged when others in the White House decided to compromise rather than wage an all-out fight that would risk defeat.

Buchanan's colleagues say he was also surprised to discover the limited direct access he would have to the president. But Buchanan is "not unhappy" and is looking forward to this fall's resumption of the campaign for Reagan's tax revision plan, a senior official said.

Regan has made himself the president's top assistant, but it is clear to other officials that his standing is shared by national security affairs adviser Robert C. McFarlane, who played a central role in managing the Flight 847 hostage crisis and will again in the preparations for November's summit with Soviet leader Mikhail Gorbachev.

Another senior official, presidential spokesman Larry Speakes, has enjoyed increasing stature under Regan. This was evident at the time of the president's surgery for a cancerous intestinal polyp, when Speakes was one of the few people, besides Nancy Reagan and the chief of staff, who saw Reagan on a regular basis.

White House officials said in interviews last week that other staff changes are also expected.

Cabinet Secretary Alfred H. Kingon, who came with Regan from the Treasury Department, is expected to be given enhanced responsibilities for domestic policymaking, and White House counsel Fred F. Fielding, who has long talked of leaving, is expected to depart this year, officials said.