President Reagan paid tribute yesterday to the "great happy warrior," Hubert H. Humphrey, whom he posthumously awarded the Congressional Gold Medal in a Rose Garden ceremony attended by prominent Democrats.

" . . . His greatest contribution to his country was his life, a life that affirmed the vitality of democracy, affirmed the fact that the democratic process is alive and full of movement and action and great plans and decent dreams," Reagan said.

The president presented the medal to Muriel Humphrey Brown, widow of the former vice president, who thanked Reagan for a "beautiful message and summation of Hubert's goals and life." She said of Humphrey: "He was one man who . . . made a great change in the life of our country."

She completed Humphrey's Senate term after his death.

Reagan praised Humphrey as "a liberal who was an internationalist, a liberal who understood that America was great and has serious responsibilities in the world, a liberal who was strongly anti-Communist.

"He loved justice; he believed our Constitution is a living document that is reborn every day," Reagan said. "He was a passionate democrat -- small 'd' and big 'D' -- who tried to make the world better according to his lights. And no one was better than he at infusing his followers with a fighting spirit."

After his speech Reagan shook hands with the front row of relatives and family friends, among them Joan Mondale, wife of Democratic presidential nominee Walter F. Mondale, a Humphrey protege.

Sen. Edward M. Kennedy (D-Mass.), a guest, told reporters afterward that Reagan had delivered "a gracious tribute" that went beyond partisan considerations.

When Humphrey, an ardent supporter of New Deal and Great Society domestic social programs, was elected vice president in 1964, Reagan was a spokesman for Republican presidential nominee Barry Goldwater. Reagan's favorite theme in 1964 and in subsequent Reagan campaigns was that government had grown so much that it menaced the U.S. economic system and American freedom.

But Reagan spoke in Humphrey's behalf in 1948 when both men were Democrats and Humphrey was mayor of Minneapolis running for the Senate against Sen. Joseph Ball (R-Minn.). In a union-sponsored radio speech, Reagan said the reelection of President Harry S Truman and the election of Humphrey were essential to stop "Republican inflation" and continue the policies of the New Deal.

It was the last time that Reagan supported a Democrat for president.

Yesterday Reagan recalled what many consider Humphrey's finest moment in politics, a speech that triggered a southern segregationist walkout from the 1948 Democratic national convention.

"In 1948, he touched the conscience of his party when he took the floor of the Democratic convention to make a passionate appeal for civil rights," Reagan said. "In the sixties, he was deeply involved in the struggle that followed U.S. commitment to Vietnam."

Humphrey's support for U.S. involvement in that war became a focal point of the 1968 presidential campaign, in which he won his party's nomination after a bitter struggle but was narrowly defeated by Republican Richard M. Nixon.

That election and the next one left a legacy of Democratic division on which Republicans capitalized. In 1980, Reagan cultivated the support of "neoconservatives," mostly former supporters of Humphrey and Sen. Henry M. Jackson (D-Wash.), who supported Democratic domestic social programs while favoring strong defense policies.

Yesterday, however, Reagan emphasized the personal rather than the political, praising Humphrey's "truly buoyant civility" and poking gentle fun at his reputation for never having had "an unuttered thought."

"His passing left Washington a lesser place," Reagan said. "He left a big silence behind him."