President Carter's "windfall profits" tax proposal received a mixed, but generally favorable reception in the House Ways and Means Committee yesterday, with members indicating they may stiffen the plan somewhat before sending it to the House floor.

On the opening day of hearings on the measure, Rep. Al Ullman (D-Ore.), chairman of the panel, told Treasury Secretary W. Michael Blumenthal, who appeared as a witness, that "hopefully in some respects" the proposal "might be tightened."

Ullman also rejected calls by liberals to consider measures to reduce oil companies' tax breaks at the same time that the panel votes on the "windfall profits" tax, instead of taking up the windfall measure first, as Ullman has proposed.

The panel's reaction to the Carter plan essentially was a expected. Democrats appeared generally to endorse the president's package, with some reservations, while Republicans called for a "plowback" provision to allow the oil companies a tax break for new exploration.

In a new twist, however, several members raised questions about Carter's statement in Iowa last Friday that, even after proposing decontrol, he would sign legislation to extend federal ceilings on oil prices if Congress were to pass such a law.

Reps. Cecil Heftel (D-Hawaii) and Bill Gradison (R-Ohio) suggested indirectly they feared many lawmakers might be on the spot if they supported decontrol and the accompanying "windfall profits" tax and then Carter reversed himself and agreed to extend price ceilings.

"The fact that he (Carter) said it (that he would sign legislation to extend price controls), we know we're in trouble," Heftel told the secretary. And Gradison asked at one point: "Are you really for decontrol?"

Blumenthal, however, joined other administration officials in asserting that what Carter really meant was that he would only sign such legislation if it also would help spur production and cut consumption - a situation the secretary insisted is not likely to occur.

Blumenthal assured the panel that Carter "is completely committed to decontrol." High administration officials made similar assertions to congressional leaders at a White House meeting on Tuesday in the wake of the president's Iowa statement.

Although some problems still lie ahead before the panel finally agrees on how to shape the tax, officials generally were encouraged by the committee's reaction to the legislation yesterday. Even some of the Republicans appeared to favor some sort of windfall tax proposal.

Blumenthal adamantly opposed the notion of including any kind of "plow-back" provision, however, asserting that "plowback should be recognized for what it is - a subterfuge for repealing the windfall profits tax." And he backed Ullman's plan to take up the windfall tax before other legislation.

Secretary also was more cautious than the White House in describing the impact of the president's tax, which most analysts regard as mild. Blumenthal said the measure would capture "some" of the extra profits on existing wells and "a portion of" any windfalls from Arab oil price boosts.

Earlier, the administration had been chided by Rep. Barber B. Conable (R-N.Y.), ranking minority member of the panel, who complained there had been "some confusion" about the proposal, "mainly because of expectations that were created by presidential rhetoric."

Conable called the proposal "an appropriate tax to be considering." But, he said, "it is the position of my side that the windfall profits tax should not be used as a windfalls in government revenues . . . to provide more energy." He called for a "plowback" provision to help spur more production.

The administration also was chided by liberals, mostly junior members of the committee, for lifting oil prices in the first place. Rep. Thomas J. Downey (D-N.Y.) told the secretary: "Decontrol is carter is Carter's greatest political mistake, and may in fact cost him his presidency."

The panel is scheduled to take testimony from other administration witnesses this morning and then question industry and consumer representatives late next week. Ullman said he hopes to send the bill to the floor in time for House action in late June. CAPTION: Picture, Treasury Secretary W. Michael Blumenthal in his appearance before the House Ways and Means Committee yesterday. By James K. W. Atherton-The Washington Post