President Carter will send to Congress next week "detailed specifications" for legislation that would impose a "windfall profits" tax on the oil industry, White House press secretary Jody Powell said today.

Briefing reporters here near Sapelo Island, where President Carter is vacationing, Powell said that Extensive White House consultation with members of Congress has reconfirmed administration optimism that such a tax will be enacted this year.

"Our consultations with the Hill continue to show increasing support for the tax," he said.

The president said that he would seek enactment of the tax two weeks ago in a nationally televised speech in which he announced his decision to order the gradual decontrol of domestic oil prices beginning June 1. He said he would propose a 50 percent tax on the profits the oil companies achieve as a result of decontrol, with the money to go into an "energy security fund" to assist low-income families and finance energy research and development.

Powell said congressional commitee aides had asked the White House to submit specifications for thhe legislation Carter wants, rather than a draft bill. He said this is a common practice when dealing with detailed tax legislation, which ends up being written by the committees in any event.

Despite considerable skepticism on Capitol Hill, White House aides have been optimistic about the chances for passage of the tax. They have argued that since Carter has already used his authority to order decontrol, the only choice facing Congress is whether to allow the oil companies to keep all of the additional profits from higher prices or only part of them.

In that context, the Carter aides have confidently contended, Congress will have little political choice but to enact some kind of "windfall profits" tax.

In addition, the president has made it clear that he intends to hammer away publicly on the issue, seizing it as a "populist" theme with broad political appeal.

However, while While House aides have continued to express optimism about chances for passing the tax proposal, they recognize that congressional powers such as House Ways and Means Committee Chairman Al Ullman (D-Ore) and Senate Finance Committee Chairman Russell Long (D-La) will have the final say on whether such a tax comes close to matching what Carter recommends.

The optimistic White House projections on the coming congressional debate received support Wednesday from Ullman.

"I think we will be able to put in place a windfall profits tax that will give us most of the revenues the president is recommending," Ullman told a news conference in Salem, Ore. He predicted the Ways and Means Committee will report out the legislation for a floor vote by the August congressional recess.

Ullman also told the news conference that any further disruption in foreign oil supplies to the United States this year would likely result in mandatory gasoline rationing. Asked about this today, Powell said there has been no change in the White House position on rationing, which is that it is not necessary now and would be ordered only in an "extreme situation."

Powell said Carter discussed the "windfall profits" tax by telephone today with his domestic policy adviser, Stuart Eizenstat, who is heading a White House task force on the issue. He said the initial administration testimony on the issue is scheduled to come from Treasury Secretary W. Michael Blumenthal May 9 before Ways and Means.

The president and his wife, Rosalynn, spent their eighth and final day of vacation on Sapelo Island today. They are scheduled to leave the Georgia coast Friday for a one-day visit to their home in Plains.

On Saturday, they will travel to Calhoun to be present Sunday at the christening of their granddaughter, Sarah Rosemary. They are scheduled to return to Washington Sunday afternoon.

The president's daughter, Amy, returned to Washington on Monday for school.