An unusually large cast of players --

48 House members and nine senators -- are scheduled to begin marathon negotiations today over the controversial Republican plan approved by the Senate last week that would mandate a balanced federal budget by 1991.

What is expected to be a month-long, contentious series of meetings comes as the Senate continues work on a much smaller deficit-reduction effort -- the $19 billion in spending reductions required for this year.

At issue in the House-Senate conference today is a Senate-approved plan to set fixed targets for deficit reductions through fiscal 1991 and require the president to impose proportional cuts in spending to meet the targets if Congress falls short. The plan, which would reduce deficits from $180 billion to zero in annual installments of $36 billion, was approved overwhelmingly by the Senate and endorsed in principle but not detail by the House.

Conferees include many of the most powerful figures in both chambers, a reflection of the radical changes the proposal would make in White House and congressional handling of fiscal policy.

Proposed modifications from House Democrats are expected to include allowing greater spending flexibility during recessions, limiting presidential discretion in making cuts, and requiring more cutbacks before next year's elections, when Republicans are expected to be hard-pressed to retain control of the Senate.

House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.), in a speech prepared for delivery last night in Philadelphia, said Democrats "want to make sure that this bill will not turn a future recession into a depression, that it will not turn the United States armed forces into the United States strategic nuclear forces, that it will not make the Congress an unequal branch of the government and that it will not turn America's struggling poor into America's abandoned poor."

President Reagan stepped up pressure yesterday for approval of the Senate-drafted plan, calling it "the last best hope to come to grips with the budget deficit." In a campaign appearance in Idaho for Sen. Steve Symms (R), he said it was "not a gimmick or a trick," as critics have charged, and called on the conference not to "scuttle" it.

While attention focuses on the plan for the future, $55.5 billion in proposed deficit reductions for this year remain far from enactment.

The Senate is expected to begin debate today on "reconciliation" legislation incorporating reductions amounting to $19 billion, achieved largely through permanent cutbacks in specific programs. Its start on the bill was delayed when senators were slow to return from their long Columbus Day recess, prompting an exasperated warning from Majority Leader Robert J. Dole (R-Kan.).

Yesterday was "a good example of why we'll probably be here till Christmas," said Dole. Only a few hours earlier he said the Senate, which had been planning to leave by Thanksgiving, would be lucky to get home by Dec. 10.

Most of the rest of the Senate's proposed savings are contained in fiscal 1986 appropriations bills, which are also moving at a deliberate pace through the Senate.

Although the House has done better on appropriations bills, it has yet to act on its package of "reconciliation cuts," in part because of a dispute over whether they meet the established targets.