Richard M. Nixon, who left Washington 3 1/2 years ago on Air Force One, was on his way back last night on United Air Lines Flight 54.

The former President returned to honor Hubert H. Humphrey at the Capitol today and is scheduled to return to his San Clemente, Calif, home Monday at 11:45 a.m., friends of the former President said yesterday.

The two men had been the bitterest of political enemies. But in the end, with one dying of cancer and the other exiled in disgrace over his part in the Watergate scandal, they had talked to each other on the phone several times to reminisce about the "brotherhood of politics" they shared, friends said yesterday.

Thus it is not so surprising, according to friends of Nixon, that the former President decided to come back to Washington to "pay his respects" to his longtime opponent. Except for a trip to China, at the invitation of the Communist government in 1976, Nixon has been a near recluse since being forced from office on Aug.9, 1974.

Nixon hoped his trip to the Humphrey ceremony could be "carried out in as quiet and dignified a manner as possible," one of these friends said.

"And he hopes the trip won't be misinterpreted. The President wanted to pay his respects because Mr. Humphrey was a former Vice President and a former adversary," the friend added.

The two men had been in touch several times over the past year or so, according to persons close to each. Nixon had called Humphrey after the Senator's cancer operation last year and during a subsequent hospitalization. Humphrey had called Nixon on Christmas Day and again just last week on Jan. 9, the former President's 65th birthday, they said.

The calls were described as "warm and filled with reminisences" by one Nixon friend. "It's not true that they were bitter enemies," he said. "They were old combatants but they also belonged to the same brother hood of politics. They would talk about the times they'd been through.

"They hammered each other in public through the years. But in the end they were civil, even warm," he added.

A Humphrey aide, in noting the exchange of phone calls, said yesterday, "The senator does not make enemies."

"The former President has been very kind and thoughtful in staying in touch with the senator during his illness," Humphrey's press secretary Betty South said yesterday.

Nixon also contributed $1,000 to a fund for a Humphrey Institute of Public Affairs at the University of Minnesota.

While it could not be immediately learned whether, Nixon had been formally asked to attend the invitation only memorial service, the former President, "will be most welcome guest," South said.

Nixon left Los Angeles on the regularly scheduled United Airlines Flight 54 about 2 p.m. yesterday for the 4 1/2 hour flight. He shared the first class compartment of the plane with 12 Secret Service agents and a CBS-TV film crew, according to a spokesman for the airline.

The spokesman said Mrs. Nixon did not accompany the former president, apparently because she had been advised not to by her doctor. He aid Mrs. Nixon had called Mrs. Humphrey aboard Air Force One earlier in the day to explain the situation and offer her condolences.

The spokesman said United first learned of Nixon's plans about 9 a.m. yesterday when reservations word made for Nixon, either by the former President or someone in his party, the spokesman said.

Because of the Nixon request, the spokesman said, Nixon and his entourage were driven into Los Angeles airport through a special gate up tp a side door that led to the departure gate. Nixon walked directly onto the plane without the necessity of going through the terminal or standing in line.

Cocktails, a snack and dinner were scheduled to be served on the flight. A spokesman said the pilot and flight attendants were advised "as a matter of information about the circumstances regarding their passenger when they arrived for their flight.