On the day before he died, Sen. Hubert H. Humphrey signed a letter urging a marriage of necessity between the Smithsonian Institution and the Museum of African Art.

Giving its blessings, the Smithsonian's board of regents voted at its Monday meeting to be "fully supportive" of the proposed union. The Congress is expected to act soon.

Humphrey, who served the Smithsonian as regent and vice chancellor between 1964 and 1968, also was the first board chairman of the financially strained private museum on Capitol Hill. "He knew we couldn't sustain our educational programs much longer without federal appropriations," said Warren Robbins, the museum's founder and director. "Whenever we asked, he helped."

"Back in 1965 when Warren Robbins was just starting the museum, he would tell me who had money, and I would say, 'Let's go get it,'" said Humphrey in 1971.

The museum, the only one of its kind in the country, owns 7,000 works of art, an extensive archive, and a row of houses on A Street NE. It assets have a value of around $7 million, but it has no endowment.Its budget is now nearing $750,000 a year, and Humphrey agreed with Robbins that the federal government was the only likely future source for such operating funds.

A move to place the small museum beneath the sheltering umbrella of the Smithsonian was initiated, in an informal conversation between Robbins and Smithsonian Secretary S. Dillon Ripley, at the opening of the Hirshhorn Museum and Sculpture Garden in 1974. "I asked him," said Robbins, "if he wanted another museum."" The campaign was one that Humphrey, the experienced bureaucrat and legislator, helped orchestrate from the start.

In 1975, the Board of Regents sent a team of accountants, museum specialists, security personnel, and scholars to examine the museum, its collections and its books. "They saw that it was good," said Charles Blitzer, the Smithsonian's assistant secretary for history and art.

Then roubles intervened. When, following a General Accounting Office audit and Senate hearings, relations between the Smithsonian and the Congress appeared to grow strained, Robbins and the regents took a different tack. The Smithsonian can accept any gift it wants, but it needs congressional appropriations to run its museums. All concerned agreed it would without first drumming up congressional support. So Humphrey went to work.

Last year he circulated a "Dear Colleague" letter urging his fellow senators to lend their support to the proposed Smithsonian takeover. A similar letter was circulated in the House by Reps. John Brademas (D-Ind.), John Conyers (Mich.), and Frank Horton (R-N.Y.), all of whom are members of the museum's national council. Their petition, urging the regents to accept the gift of the museum, was signed by 36 senators and 120 members of the House.

Humphrey then let it be known he would introduce a bill "authorizing" the Smithsonian to accept the gift. The regents took action on Monday.

Humphrey had hoped to introduce his bill when Congress reconvenes. On Thursday, knowing he was weakening, Humphrey wrote Sen. Wendell Anderson, his Minnesota colleague, asking Anderson to submit the bill instead. "We will do so in the next week or 10 days," said an aide to Anderson. "We are lining up co-sponsors. And we felt it best to wait until the Senate had completed its memorial service for Sen. Humphrey."

Should the transfer be approved, the operations of the museum (if not the uniforms of its guards) will not quickly change. The museum, for the foreseeable future, would remain on A. Street NE. "My guess is that it would go on for a very long time pretty much as it now is," said Blitzer. "It looks to me," said Robbins, "as if this thing will go through."