The new windfall profits tax that President Carter proposed last Thursday tends to support what many analysts have been saying for weeks: The plan is not designed primarily to siphon profits from the oil companies, but rather to provide Carter with a cover for his earlier decision to decontrol oil prices.

Although the president made a big show of doing battle with the oil companies -- nd with supposed other "critics" of his new plan -- the measure's tax bite is so modest that almost no one really opposes the legislation except for a handful of liberals who think it ought to be tougher.

Even the oil industry isn't fighting the measure seriously. One oil industry executive confessed last week his company was grunting about it merely to save face for Carter and House and Senate lawmakers -- to avoid pushing them into a really serious tax.

Despite all the administration's rhetoric, Carter won't have any trouble getting the bill through Congress, the oil executive conceded privately last week. "Hell," he said, "the president is pushing on an open door." One proof is that oil-state legislators aren't seeking to block the plan.

The modest size of Carter's tax plan was especially intriguing in light of his campaign about profits levels. In speeches recently, Carter repeatedly has stressed the need to prevent the oil companies from reaping a "huge bonanza." He pledged a tough tax that would sap "50 percent" of the total windfall.

But Treasury figures show that in 1982 -- the first year oil prices are fully decontrolled -- the windfall profits tax would reap only $1.7 billion more in U.S. taxes than the $4.3 billion that the industry would have had to pay anyway under existing federal income tax -- on extra revenues of $14.5 billion.

In fact, between now and the end of 1982, the tax would generate a net increase in federal receipts of $3.75 billion, which is a scant 11.4 percent of the $32.9 billion "windfall" that the White House estimates the oil companies will get over the period.

(The White House contends the 50 percent figure ultimately is achieved if you also add in the extra federal income taxes the industry would have to pay on its "windfall." But the oil companies would have had to pay most of that anyway.)

Moreover, the tax itself is riddled with a series of big exemptions: old oil obtained from "marginal" wells, deep wells that produce less that 35 barrels a day, oil from new production wells, oil from Alaska and oil from so-called "enhanced recovery" -- these wouldn't be hit at all.

The revenues are eroded further because the windfall tax on profits from oil discovered before 1972 is phased out quickly tax first and then dealing with the rest of the proposals separately.

Carter also skirted the controlversial issue of what the windfall profits revenues would be used for. The president proposed spending $800 million a year on rebates to low-income families and several hundred million more on energy research and mass transit. But he left much of the program vague.

Still, Carter faces some serious problems in Congress:

Although the profits tax may pass the House intact, it may well be watered down when it gets to the Senate, where Finance Committee Chairman Russel B. Long (D-La.) is planning to tack on a "plowback" provision allowing exemptions for money the oil firms use on new exploration.

Carter has vowed to oppose any plowback clause, but the Senate is likely to pass one. And with the windfall profits tax netting a mere $1 billion to $1.7 billion a year in new revenues, the president doesn't have much to give away.

If the profits tax passes, Carter still faces the battle over what to do with the new revenues -- and everyone in Congress seems to have his or her pet project: Liberals want bigger rebates for the poor, New England legislators want more subsidies for heating oil and others want a payroll tax rollback.

There also is the question of what to do about Carter's list of newly promised tax credits. Tax experts say most of them can't be justified except as possible rewards for fence-sitting legislators (a planned tax credit for using wood-burning stoves is an example). Yet, the bills could provoke a fight.

Administration officials argued last week that one major justification for the tax proposal is that it provides a vehicle for protecting low-income families from the higher fuel costs that will stem from oil-price decontrol. Without the Carter plan, the $1.7 billion would go to the oil firms.

But to many observes, the major benefit of the tax can be seen by looking at 1980, not 1981 or 1982, as the White House continues to suggest: Lashing out at the oil companies is good politics for Carter after he has moved to decontrol oil prices. Even a modest tax can help a president look "tough."

And, in the last analysis. Carter may need all the help he can get. New figures made public last week showed the administration may have been too optimistic in its estimate of the impact of oil-price decontrol. The move now may add 7 cents to the price of gasoline -- not 4 cents, as estimated before. CAPTION: Chart, Impact of Proposed Windfall Profits Tax, Treasury Department. Bill Perkins, The Washington Post