Senate opponents of the natural gas pricing bill, buoyed by unintended help from the White House, said yesterday they may be able to kill the legislation without a filibuster.

Hopes of the unusual coalition - liberals who say the bill will cost consumers too much, conservatives who say it will hamstring the gas industry - were lifted by these developments:

Sen. Russell B. Long (D-La.), who, as Finance Committee chairman, plays a pivotal role in President Carter's push for an energy package, confirmed that he will vote against the House-Senate conference agreement on gas.

Minority Leader Howard H. Baker Jr. of Tennessee, peeved by Carter's promise to spend more money on breeder reactor research to win a conferee's vote, said he will oppose the gas compromise and support a filibuster if need be.

Two other conferees who signed the gas agreement, Dale Bumpers (D-Ark.) and Mark Hatfield (R-Ore.), indicated that they, too, now will oppose the measure if the president's anti-breeder position has changed.Both have been key supporters of Carter's earlier fight to stop construction of the Clinch River breeder reactor in Tennessee.

The anticompromise leaders quickly won the support of 18 liberals and conservatives for a plan to send the bill back to conference - that is, kill it - when debate begins after the Labor Day recess.

Although they had no headcount, leaders of the opposition said enough of a "groundswell" of doubt is rising in the Senate that they may be able to stop the pending bill without resorting to a filibuster.

The administration moved to try to prevent further erosion of support for the controversial compromise, but one result was more confusion over the White House position.

Energy Secretary James Schlesinger sent a letter to members of the Senate Energy Committee, providing details of the agreement betwen the president and Sen. James McClure (R-Idaho).

Spokesmen for Schlesinger said that McClure got it wrong. McClure said he got it right, without question. Other senators said they knew only what they read in the newspapers - and were fuming over the idea of a presidential deal for a crucial vote.

The furor gave critics of the administration price deregulation scheme more fuel and growing hope that they can scuttle the bill, a prospect that seemed remote two weeks ago.

Sens. James Abourezk (D-S.D.), Howard Metzenbaum (D-Ohio) and Clifford Hansen (R-Wyo,) said their alternate proposal would require the conferees to report out a scaled down, less costly and less complex regulatory bill.

Their alternate would avoid the complicated price deregulation formula in the pending bill, provide authority for gas allocations during emergencies and allow needy states to get surplus gas from producer states.

The opponents' hopes for defeat of the compromise, which took conferees eight months to hammer out, soared with a bitter reaction in the Senate to the president's effort to win the key vote last week of McClure.

Carter, according to McClure, agreed to a three-year funding authorization of $1.5 billion for additional research on the controversial breeder reactor program.

Neither the White House nor the Department of Energy took issue publicly with McClure's report Wednesday, but administration sources insisted that the president had merely reiterated his previous position.

McClure said that he talked with Schlesinger yesterday, but Schlesinger did not challenge the accuracy of the senator's understanding of the agreement with Carter.

"The details were worked out over the weekend with Department of Energy people and on Monday and Tuesday - with meticulous care," McClure said. "There has been no discussion that my understanding was erroneous."

McClure added, "They are trying to mollify people on both sides of this - Sens. Baker and Jim Sasser (D-Tenn.), who are interested in Clinch River, and Sens. Hatfield and Bumpers, who oppose it."