As jubilant Republicans convened in Washington to toast their November victories, Democrats gathered today to search for ways to coax white voters back into the party fold in the once solid South.

With mass defection by white voters last fall, southern Democrats saw President Reagan sweep through their region and lay the foundation for more permanent Republican gains.

"If we don't get together and work together, we're going to be the minority party," Kentucky Democratic Chairman Ed Coleman warned.

But the prescriptions laid out reflected tensions in the party, especially in the South. White southern chairmen said Democrats need to reach out to white voters while maintaining overwhelming support among black voters, the most loyal constituency in the party.

One black elected official later described that as little more than "the illusion of inclusion."

Blacks here made clear that the Republican Party holds little attraction for them but they are not content to be taken for granted by the national Democatic Party.

Today's meeting was called by southern blacks, who then invited the southern Democratic state chairmen, many of whom fought for civil rights gains but now are fighting the erosion of white voters.

"We have to go back and capture our white constituency," said Texas Democratic Chairman Bob Slagle.

Other white party leaders echoed him and blamed the party's national image for white voters' shift.

"If our agenda is to win elections in the future, the image we have to project has to be more moderate than it has been in the past," Alabama state Chairman Jimmy Knight said.

Southerners, Jesse Bankston of Louisiana said, "are saddled" with the national party's image, which he said makes it increasingly difficult for Democrats to attract "professional, middle-income" voters.

Party officials admired Republican successes of last fall. "In North Carolina, they won the gold, the silver and the bronze," said state Chairman Wade Smith. He was referring to victories by Reagan, Sen. Jesse Helms (R-N.C.) and Rep. James G. Martin, now governor.

But Democrats here laid some of the blame on their own party. Said South Carolina's Bill Youngblood, "The Democratic Party has defied the law of physics. We are less that the sum of our parts."

Virginia party Chairman Alan Diamonstein said that the GOP laid "claim to the American mainstream" in 1984 and Democrats had to recapture it to become successful in presidential elections.

Atlanta Mayor Andrew Young, one of the hosts, said Republicans had succeeded in the South because they had "out-organized" the Democrats. Another host, Birmingham Mayor Richard Arrington, said that blacks must communicate that "they are concerned about more than welfare . . . . We must deemphasize some of the racial rhetoric."