Democratic congressional leaders agreed yesterday to press for prompt enactment of President Carter's "windfall profits" tax on oil, but were unable to work out a specific plan on how to deal with a series of companion proposals.

In a breakfast meeting, the leaders decided on a timetable that would aim for House passage of the measure before the July 4 recess, with final action by the full Congress in time to send it to Carter's desk by mid-august.

However, Sen. Russell B. Long (D-La.), chairman of the Senate Finance Committee, reportedly was reluctant to promise that the Senate would take up the tax portion of the Carter package before acting on other parts, as the House is doing.

Long was said to have agreed to explore the possibility of formally invoking cloture to stave off any floor attempts to combine the various elements into a single bill. However, he apparently declined to make any commitments.

Beside the "windfall profits" tax, Carter also has proposed companion legislation that would tighten the foreign tax credit for oil companies and establish a special fund to finance research, mass transit and tax cuts for the poor.

However, the House Ways and Means Committee has persuaded the administration to go along with taking up the "windfall" tax proposal first, to more controversial companion measures.

House leaders want the Senate to follow the same plan. Yesterday, both Long and Senate Majority Leader Robert C. Byrd (D.W.Va.) reportedly agreed that tactic would be a good one, but were unable to guarantee it could be done.

The developments came as, separately, the Ways and Menas Committee began its first drafting session on the Carter "windfall" tax proposal with complaints by panel Republicans that Democrats were trying to shut them out.

On Wednesday, Committee chairman A1 Ullman (D-Ore.) held a caucus of panel Democrats to outline his own plan for dtiffening the Carter "windfall" tax, and promised prompt hearings on the foreign tax credit. Republicans were barred.

Yesterday, Rep. Barber B. Conable (R.-N.Y.), the panel's ranking minority member, complained publicly that Republicans were "upset" about the Democrats' caucusing tactics, and warned the GOP would not cooperate if it were shut out.

The committee spent most of the day yesterday being briefed by staff members, and did not get to any decisions on the "windfall profits" tax bill. The panel is scheduled to meet again on Monday.