The Carter administration is quietly weighing contingency plans to impose event the President's proposed crude oil tax - the so-called "centerpiece" of his energy plan - is rejected by the Congress.

Energy Secretary James R. Schlesinger said yesterday that "the President will have to consider alternative measures" if the controversial tax is not passed within the next six to eight weeks.

Administration officials say that the most likely alternative to the tax would be a tariff on imported oil.

The crude oil tax is one of several steps Carter has proposed to raise the price of petroleum products and thus discourage consumption and oil imports. A tariff on imported oil would have a similar effect. A quota on imports would force down consumption more directly, by limiting supply; it thus might also require some form of supply allocation by the government.

The crude oil tax has come under fire on Capital Hill, where legislators passed one big tax increase last year - for Social Security - and are wary of passing another.

The administration says Carter has power to raise tariffs on his own, without consulting Congress.

Senior energy officials say that higher tariffs on imported oil could be levied by the President under existing authority to increase the price of domestic oil, now set at $8.00 a barrel, to the $13.50 a barrel piece of foreign oil.

The United States will pay out more than $40 billion this year to import nearly half the oil it consumes.

Schlesinger during closed door meetings with Treasury Secretary W. Michael Blumenthal and others concerned about the eroded position of the dollar in foreign exchange markets - attributed largely to the U.S. oil payments - has urged holding out for the crude tax instead of pressing for tarriffs.

One reason Schlesinger has argued so vigorously in administration councils to continue to press for the tax is that it would allow DOE to phase out its cumbersome regulatory program to equalize the costs refiners pay for different priced oil.

Schlesinger wants to end this "entitlements program," as it is known, because it in effect subsidizes oil imports which is counter to the objectives of the Carter energy plan.

In the last two weeks, however, Sen. Russell Long (D-La.), the powerful chairman of the Senate Finance Committee, issued a statement following a private Oval Office meeting with Carter that the aadministration was "beating a dead horse" while continuing to press Congress into passing the crude tax.

Blumenthal and other Treasury officials have grown increasingly unsettled by the glum prospects Long and other Congressional leaders hold out for the crude tax, and have been pressing for some kind of action on energy that will take pleasure off the dollar.

They are also said to be impatient with Schlesinger's repeated assurances the Congress will pass the tax.

"Treasury and the Fed (Federal Reserve Bank) are in one camp, and everyone else is in another," said one senior energy official asked whether there was a split within the administration on the tax.

While an import quota system including provisions to allocate oil products is also under possible consideration, energy officials say that tarriffs up to $5.00 a barrel or more would be preferrable.

Domestic oil - including price-controlled oil - sell for an average $8.00 a barrel, compared with $13.50 a barrel for imported oil.

Ironically, Schlesinger first floated the proposal for a $5 a barrel tarriff last fall to prod the Senate into passing the crude tax.

It didn't. Instead, the Senate passed an energy tax bill without the crude tax, and in its place tax incentives to increase energy production and spur conversation. The Senate energy tax bill, which has yet to go to conference, also includes an amendment proposed by Sen. Robert Dole (R-Kan.) that would strip Carter of his authority to impose tarriffs - technically called "license fees."