Rep. Al Ullman (D-Ore.), chairman of the House Ways and Means Committee, said yesterday that Congress this year probably will pass President Carter's tax on "windfall profits" resulting from the administration's plan to decontrol oil prices.

At a Salem, Ore., news conference, Ullman also said the nation could be forced into mandatory gas rationing by the end of the year. "If we have any further disruptions [in supply], I think rationing would be a very clear probability," he said.

While stopping short of saying how close the House version of the windfall profits tax would be to Carter's proposal (which would tax 50 percent of the oil companies' windfall profits), Ullman said later, "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." He said he expects his tax-writing panel to report out a bill for a floor vote before the August recess.

On an equally controversial oil-company tax issue, which has attracted little public attention, Ullman said he expects Carter's proposal to eliminate $500 million in foreign tax credits claimed by the oil industry to be considered separately from the windfall profits tax measure by the committee.

Ullman's suggestion that some kind of a windfall profits tax is likely to pass this year comes less than a wekk after a similar prediction by Sen. Russell B. Long (D-La.), chairman of the Senate Finance Committee.

Long, however, was less precise about the timing. Most congressional leaders have said that they do not think the windfall profits tax would be enacted before next year at the earliest.

Carter has proposed, in effect, a two-phase tax: one phase would tax away 50 percent of the difference between oil companies' income at current, price-controlled levels and at the new, decontrolled price; the second phase is a 50 percent tax on the difference between the current world oil price and future increases imposed by the oil cartel. "I would think that one or the other [tax], or both of them might pass," Long said.

Unlike Ullman, Long repeatedly has said that some of the tax revenues may have to be set aside for the oil industry to plow back into exploration and production.

Ullman did not offer a headcount on how his committee would vote on the windfall tax, but he said it is likely to pass because "there is broad political support in the Congress and the country" for it.

During the two-year congressional battle over Carter's April 1977 national energy plan, the House narrowly passed a similar oil measure, the crude oil equalization tax, which was rejected later by the Senate.

Unlike before, however, Carter has not coupled decontrol with passing a tax.

Carter's plan calls for decontrolling oil prices beginning June 1, allowing all controls to expire by October 1981.

In a related development, a Congressional Budget Office report released yesterday said Carter's plan to decontrol oil prices "will further acceleratee inflation, and may slow economic activity and increase unemployment."

CBO's analysis of the plan's impact differs sharply from the projected impact of decontrol published by the administration.

The White House said the cumulative impact on inflation from decontrol would be .75 percent by 1983; CBO says it will be 1.25 percent.

As for reducing oil imports, CBO says the Carter plan would have 250,000 barrels of oil a day less than the 1.1 million barrels a day in lower imports that the White House envisions by 1985.

While the White House says that producer revenues before taxation will be increased by $16 billion from the onset of decontrol until the end of 1981, CBO says the companies will earn an added $17.6 billion before taxes.

Sen. Henry M. Jackson (D-Wash.), chairman of the Senate Energy Committee, issued the CBO study yesterday with a written statement that "we get all of the inflationary impact of the president's proposals with no real benefits. The new production will be less than one-half of 1 percent of our domestic demand for oil."

Jackson, backed by liberals, has mounted an effort in the Senate to block the president's decontrol plan.

Long, a proponent of oil decontrol, gives Jackson's move little chance of success. "Well, they'll have some votes. But I doubt they'll have enough. I think that the decontrol plan is going to go through," Long said.

A similar move in the House launched by Rep. Toby Moffett (D-Conn.) and other liberals has likewise been dismissed by pro-oil-industry congressmen who say that the sentiment in the House runs against moves to block Carter's decontrol plan.