Is Carter planning a tuneup for Detroit?

With a deflating jolt akin to giving up a limousine for a subcompact, President Carter turns from the heady world of diplomatic summitry to the depressed state of the American auto industry.

High on the President's agenda upon his return from the Venice summit meeting of Western heads of state is a decision on what the federal government will do to ease the plight of this lynchpin US industry.

Some kind of relief for the carmakers -- reeling from a deepening domestic recession and an invasion of gasoline-skimping foreign imports -- seems assured.

It is an industry that no politician can ignore, particularly four months before an election. It provides one out of every 12 manufacturing jobs in the country; underpins other basic industries such as steel, rubber, and machine tooling; supports 50,000 small- and medium-sized suppliers, and touches the economy of every state.

Assistance to the industry already is shaping up as a campaign issue between Mr. Carter and probable Republican challenger Ronald Reagan.

And action is being demanded on Capitol Hill in a rare display of near-unanimity. The Senate urged help earlier this month by an emphatic margin of 90 to 4. The House, spurred by the top leaders of both parties, this week echoed the call without a dissenting voice.

The precise form of federal help remains to be determined. The President promised a group of auto-state senators shortly before departing for the European summit that he would act within three weeks, or by early July.

Whatever course he recommends, it appears likely to rule out the sort of sweeping deregulation of the auto industry being pushed by former Governor Reagan, who claims the industry "is virtually being regulated to death."

Mr. Carter is reported to have told participants in a recent meeting on the issue that neither Congress nor the public would accept lower environmental, fuel economy, and safety standards.

Election-year concern in the White House and on Capitol Hill over holding down federal spending is believed to doom any assistance requiring significant outlays of tax dollars, such as liberalizing unemployment benefits for workers idled by foreign competition.

"I'm not sure what we could realistically conceive" in additional federal financial aid acceptable to this balanced-budget-minded Congress, concedes one House strategist.

What appear to remain are middle-ground, relatively low-cost options relying on incentives to stimulate purchases of American cars and diplomatic pressure to restrain imports of foreign cars.

The President already has said, for example, that he would try to make available more credit for car buyers.

He also is being urged by Secretary of Transportation Neil E. Goldschmidt, the United Automobile Workers union, and many lawmakers to curb car and truck imports from JApan -- either by negotiating voluntary cutbacks or imposing temporary import quotas.

Others, such as special trade representative Reubin O. Askew and House Energy Subcommittee chairman John D. Dingell (D) of Michigan, recommend pressuring foreign automakers to build more factories here.

The whole issue of world motor-vehicle trade may be elevated to an "auto summit" now being suggested for this autumn in Paris, organized by the Organization for Economic Cooperation and Development.