Strom Thrumond (R.S.C.), the Dixiecrat syndicate for president in 1948, until now has opposed virtually every civil rights bill in his 23 years as a senator.

But faced with a stiff reelection battle in a state where winning black votes has become important, Thurmond last week joined 11 other senators from the Old Confederacy in supporting a constitutional amendment that would give full voting rights in Congress to residents of the District of Columbia -- a measure that had the active lobbying support of the civil rights movement and had been called by some "the civil rights act of 1978."

Thrumond not only voted for the measure, he jumped on the bandwagon early and actively supported it in floor debates.

The support of the southern senators -- including Herman Talmadge (D-Ga.) -- provided one of the most vivid illustrations to date of the new reality of politics in the South, where the number of black voters has doubled in the past 13 years since the passage of the Voting Rights Act of 1965.

The votes of the 12 southern senators were crucial to the constitutional amendment's one-vote 67-32 victory. As a constitutional amendment it needed a two-thirds majority of the voting senators. Joining Thurmond and Talmadge in voting approval were Sens. Lloyd Bentsen (D-Tex.), Howard Baker R-Tenn.), Dale Bumpers D-Ark.), John Danforth (R-Mo.), Thomas Eagleton (D)Mo.), Ernest Hollings (D-S.C.), Sam Nunn (D-Ga.), James Sasser (D-Tenn.), John Sparkman (D-Ala.) and Richard Stone (D. Fla.).

Their support was all the more striking because of the strong opposition over the years of southern conservatives toward any degree of home rule for the District of Columbia, whose population is 72 percent black.

Thurmond and Talmadge, for instance, opposed in 1960 a constitutional amendment allowing District residents to vote for president and vice president and in 1973 voted against limited home rule, with an elected mayor and city council, for the District.

Both Talmadge and Thurmond, along with most southern representatives and senators also voted against the voting rights act, legislation which made it possible for large numbers of southern blacks to register and vote.

D.C. Delegate Walter E. Fauntroy was one of the first politicians to recognize the new clout of southern black voters. In 1970, he led a busload of District residents to the South Carolina congressional district of Rep. John L. McMillan (D), the self-styled mayor of Washington. From his position as chairman of the House District Committee, McMillan ran the city like a personal plantation and was able to block home rule efforts for decades.

Drawing on the 34 per cent black voter registration in the district, a black doctor, Claud L. Stephens, forced McMillan into a runoff which Stephens later lost. Two years later, a white candidate, State Rep. John Jenrette, ended McMillan's 32-year Congressional career.

Fauntroy ran a similar grassroots campaign drumming up support for the full voting rights amendment, and this time it appeared that the old-line southerners had learned what he called the "new math" of southern politics.

In Alabama and Georgia, for instance, the number of black voters has increased 70 percent since 1964.

In South Carolina, Thurmond's home state, black were not a sifnificant political factor until 1965. Since then, though, the number of black voters has increased 50 percent, and in some regions they cast more than one third of the ballots. Throughout the state, black voters make up more than 25 percent of the electorate.

"The voting rights act affected as great a change in politics here (South Carolina) as in any state of the union. All politicians, including Thurmond, seek blacks' support," said this year's edition of the authoritative Almanac of American Politics.

Votes from the southern states also helped win passage of the D.C. voting rights constitutional amendment in the House last March.Of 102 representatives from states in the Old Confederacy, a bare majority -- 54 --voted in favor of giving the District full voting rights in Congress.

In years past, it would have been unlikely for a handful to support such a measure.

This change was a result of a delibrate campaign by civil rights leaders in the home states of senators and representatives to make a vote against D.C. voting rights look like a vote against black voting rights.

Key southern senators, for example, admitted to being besieged with phone calls from black voters in their state urging a yes vote.

"What it came down to," said an aide to one southern senator, "is a black versus white issue . . . . Take out the black issue," he added, "and people wouldn't have a problem with voting against it."

Fourteen years ago, before the passage of a law that made it possible for large numbers of southern blacks to vote, all those calls and letters civil rights advocates directed to southern senators wouldn't have made much difference.

In South Carolina and Georgia --which now have the largest percentage of black voters of any southern states -- only 17 per cent of the registered voters were black in 1964.

The growing black voting minority changed the nature of politics in those states. Thurmond, a vigorous 76-year-old, sensed the change in 1970 when he appointed his first black staff members. Later, near the end of the Ford administration, he supported the nomination of Mathew Perry, a South Carolina black leader, as a judge on the U.S. Court of Military Appeals.

Now he is running for reelection against a young, attractive candidate, Charles (Pug) Ravenel, who represents the image of the moderate new South.

Thurmond denied the increased number of black voters in South Carolina had anything to do with his vote Tuesday.

"I thought it was the fair things to do," he said. "The people in the District were the only people in the United States who do not have voting representation in Congress.

"I do not see how we can hold ourselves up an exemplary nation in the democratic process when we continue to deny to over 700,000 people the right to have voting representation in Congress."

Asked why he voted in 1970 against allowing District residents to vote for the president and vice president. Thurmond said, "that was my thinking at that time. But the U.S. has been taking a more leading role since then in espousing democracy throughout the world."

For Washington blacks, one of the Senate's arch-villains was Robert C. Byrd (D-W.Va.), the Senate majority leader who as chairman of the District appropriations subcommittee 12 years ago had consistently opposed measures that would help welfare recipients in the city.

Yet Byrd too has changed. While he opposed the voting rights act in 1965, he voted in favor of its extension five years later.

And he became a key supporter of the resolution to give District residents votes in Congress, announcing in favor of it last week and working Tuesday to secure the votes of wavering colleagues.

"This is a human rights issue," said Byrd. "How can we criticize the Soviet Union if we deny to citizens in the District of Columbia the right to vote?"

There were indications, however, that some wavering senators ending up supporting the congressional voting rights for the District in the belief that the constitutional amendment would fail to be ratified by the necessary 38 state legislatures within the required seven years.

Sen. John Sparkman (D-Ala.), a populist who has generally followed the obligatory southern politicians' anticivil rights stance, said he would not have voted for the bill if it didn't require ratification by the states.

Byrd, the Senate majority leader said in throwing his support to the constitutional amendment last Sunday, "I think it it time to let the people decide, through the Constitution" on D.C. representation.

"If it had been a secret ballot," said one liberal, northern senator who voted for it, "the vote would have been 67-32 against instead of 67-32 in favor."