Virginia Republican Sen. John W. Warner, saying a bill to establish a national holiday to honor Martin Luther King Jr. has "enormous symbolism in the black community," announced yesterday that he will vote for the measure when it comes before the Senate, perhaps as early as this week.

Warner, a conservative who so far is facing little opposition for his reelection bid next year, made the announcement during a convocation speech at Hampton Institute, a 116-year-old, predominantly black school near Norfolk.

"I don't want this to be just another holiday, a day of leisure," Warner said in an interview after the speech. "I asked for a commitment to America," he said, "to work together to see this a day dedicated to unity and for striving for all citizens irrespective of race, creed or color . . . and a greater degree of justice for all citizens."

Warner declined to discuss the political impact of his announcement. The Virginia General Assembly consistently has defeated efforts to establish a state holiday honoring King.

"These are the tough ones that members of the Congress have to call now and then," Warner said.

Although Democratic Gov. Charles S. Robb and the state party have yet to settle on a major candidate to oppose Warner, the Democrats are clearly counting on anti-Reagan and Republican votes from the black community in an effort to unseat Warner, who is completing his first term.

Warner had planned for at least several days to make the announcement at the school but did not mention it Saturday during his annual Atoka Farm Supper that attracted about 3,000 Republicans to his home near Middleburg.

The bill, which would set aside the third Monday in January in King's honor, overwhelmingly passed the House, 338 to 90, early last month. The outcome in the Senate has been less certain, but support for the bill has gained in recent weeks as conservatives have eased their opposition.

Passage of the bill could take away a major rallying cry being used in black voter registration drives that are thought to benefit the Democrats. The bill was a major focus of the 20th Anniversary March on Washington late last month that drew more than 300,000 persons.

Warner said he talked with Republican Senate Majority Leader Howard Baker of Tennessee on Friday and predicted the bill will pass "with few, if any, dissenting votes expected." The bill would go into effect two years after its enactment if signed by President Reagan, who has opposed the measure partially on economic grounds.

In the interview, Warner said his speech cited estimates that the holiday, which would become the 10th in federal law, may cost the economy about $4 billion in lost productivity from federal, state and private industry.

"It's difficult to declare another day of nonwork," Warner said, also noting the nation's continuing high unemployment. "But against those negative aspects are the potent positive aspects. [The bill] would not only honor Doctor King but the leadership of the black community that has contributed to the citizenship of our nation."

Warner said the cost of the holiday would be offset by "the inspiration for more black people to achieve . . . and become members of the work force and become taxpayers. The loss will be paid many times over by their participation."

Aides to Republican Sen. Paul S. Trible, who was elected last year, said yesterday that Trible has not publicly announced how he would vote on the measure. Trible could not be reached for comment.

In the House vote last month, the six Republican members of Congress from Virginia split their votes on the measure, with Northern Virginians Frank R. Wolf and Stan Parris and Thomas J. Bliley Jr. of Richmond voting in favor of the bill. Opposed were Herbert H. Bateman of Newport News, G. William Whitehurst of Virginia Beach and J. Kenneth Robinson of Winchester.

Virginia Democrats voting for the bill were Norman Sisisky of Petersburg, James R. Olin of Roanoke and Frederick C. (Rick) Boucher of Abingdon. Democratic Rep. Dan Daniel of Danville opposed the measure.