A total ban on tobacco advertising, as proposed Tuesday by the American Medical Association, would face a severe constitutional challenge requiring strong evidence that it would cut tobacco consumption and that no milder measures would work, legal experts said yesterday.

Without such evidence, they predicted, such a ban probably would be ruled unconstitutional by the Supreme Court, which in recent years significantly has broadened First Amendment protections of freedom of speech in advertising.

Among those claiming that a ban would be unconstitutional yesterday was the American Civil Liberties Union, which pledged to testify against any bill introduced in Congress to prohibit tobacco advertising.

"In a society which believes in the First Amendment, you can't allow information control to become a means of behavior modification," said Burt Neuborne, the ACLU's legal director.

The movement to ban the ads has picked up considerable momentum in the past few months. In addition to the AMA's action on Tuesday, the American Heart Association called for a ban last month, while the American Lung Association did so several years ago. The American Cancer Society plans to urge limiting the location and content of the ads, according to Michael Pertschuk, former chairman of the Federal Trade Commission and a member of the society's committee on smoking and health.

Pertschuk said there is scientific evidence that "cigarette advertising both encourages younger people to take it up and reinforces the addicted smoker, but it's not conclusive."

He said Congress would have to weigh this evidence, along with the health hazards of tobacco, in considering an advertising ban.

"That's a weighing Congress is perfectly capable, constitutionally, of performing," he said.

Such evidence, legal experts said yesterday, would be crucial in any Supreme Court test.

The Supreme Court's position on freedom of speech in advertising was defined most clearly in a 1980 decision, Central Hudson Gas and Electric Corp. v. Public Service Commission of New York. The court ruled that it is unconstitutional for the commission to prohibit electric companies from advertising to raise the consumption of electricity, even though the court agreed with the goal: conserving energy.

The majority opinion, by Justice Lewis F. Powell Jr., said the government could limit "commercial speech," or advertising, only if it could be shown that the limit effectively would further a "substantial interest" of society, and only if the benefits could not be achieved by less restrictive measures.

In the context of tobacco, lawyers said yesterday, this probably would require showing that advertising increases tobacco consumption and that a total ban would reduce consumption more effectively than such less drastic restrictions as increased health warnings.

There is considerable debate on both these issues.

Kirk Johnson, the AMA's general counsel, said that a ban on tobacco advertising would stand up in court because tobacco's damage to health is well documented and evidence exists that advertising increases its consumption.

He said the most difficult point to prove would be that less restrictive measures would not work as well as a total advertising ban.

"That's where we're going to have the battle," Johnson said.

The court has an opportunity to clarify the issue further next year, when it is expected to rule on the constitutionality of a law prohibiting casino advertising in Puerto Rico, where gambling is legal.

A congressional ban on cigarette advertising on television and radio went into effect in 1971, and was upheld that year in the U.S. District Court in Washington. The Supreme Court never has reviewed the constitutionality of the ban.

Some experts questioned why the AMA had not chosen to try to outlaw sales of tobacco. Johnson said that such a move would be unrealistic, because many of America's 50 million smokers would be driven to buying tobacco illegally.

"You'd have the worst kind of violation of federal law ever seen," he said.

Johnson added that the AMA believes in education, not coercion.

"The AMA is not going to make people do things that are healthy," he said.