Democratic Party leaders yesterday cheered their candidate's upset victory over a Ronald Reagan Republican in a Mississippi congressional election, but differed among themselves on the timeliness of attempting to discipline southern Democrats who have been voting with Reagan in the House. d

Party Chairman Charles T. Manatt, calling the special-election victory of Democrat Wayne Dowdy in Mississippi's 4th Congressional District a signal to wavering Democrats, told reporters that "it is high time" some of the dissidents were disiciplined by the House Democratic Caucus.

But Manatt ran into a mixed reaction when he met later with House Democratic leaders. Majority Leader James C. Wright Jr. (D-Tex.) said he and Speaker Thomas P. (Tip O'Neill Jr. (D-Mass.) were inclined "to be fairly tolerant" toward the Dixie Democrats whose votes have given Reagan his citories on the early budget tests. Majority Whip Thomas S. Foley (D-Wash.) said flatly, "We are not considering discipline at this time."

House Democratic Caucus Chairman Gillis W. Long (D-La.) said, however, that he would call a meeting of all House Democrats to consider a disciplinary motion proposed by Rep. Toby Moffett (D-Conn.) and other liberals.

Manatt singled out for criticism Rep. Phil Gramm (D-Tex.), co-sponsor of the budget resolutions Reagan and the Republicans pushed through the House over the opposition of the Democratic leadership.

Manatt said Gramm has participated in strategy session of Budget Committee Democrats and then plotted acitively with the Administration to thwart his own party's position.

Manatt called that "an abuse of responsibility," and urged caucus discipline against Gramm and any other Democrat "with a consistent pattern of actions" against the party positions.

But Wright suggested that disciplinary action would have to wait. "In due course, at the beginning of the next Congress, we will elect people to committees," he said, "and it is predictable that certain people will not be elected to leadership committees' like Budget, Ways and Means or Rules. "But meantime," he said, "we will extend the olive branch to as many as will grasp it."

Officials pointed out that removing Gramm from the Budget Committee could be initiated by the caucus but would have to be approved by the full House, where Republicans and southern Democrats might block it. Other moves, including censure or suspension from the caucus, could be taken by the caucus.

Gramm rejected Manatt's complaints, saying, "I think millions of Democrats, especially in the South and West, are going to be surprised that the chairman of the Democratic Party seems intent on making fiscal respondibility and a commitment to balance the budget partisan Republican issues, and seeks to punish those within the Democratic Party who have taken leadership positions in carrying out a mandate which is clearly supported by the majority of the American people."

While this argument rumbled, Democrats celebrated their first win in a previously Republican district since Reagan became president. The victory of Dowdy, 37, the mayor of McComb, over Republican Liles Williams in a district the GOP had held since 1972 was a distinct upset. Williams had led in the first primary, outspent Dowdy by 4 to 1, and flooded the district with endorsements from Reagan.

Downdy, who criticized Reagan's proposed Social Security cuts and supported extension of the Voting Rights Act, won by an unofficial 1,100-vote margin in a heavy turnout of 110,000. He said near-solid support from blacks played a "very prominent role" in his victory.

O'Neill and Chairman Tony Coelho (D-Calif.) of the Democratic Congressional Campaign Committee said the victory showed that voters did not want "rubber stamps for Reagan" in the House. But Dowdy, noting Reagan's popularity in his district, said he would be independent of pressures from either the Democratic leadership or the White House.

Last month, Democrats came within 300 votes of an upset victory in a strongly Republican district in Ohio, and earlier in the year each party had retained a seat in special elections in Michigan and Maryland.

The Mississippi vacancy was caused by the Resignation of Rep. Jon C. Hinson (R) after his arrest on sodomy charges in a House office building restroom.