It was part teach-in, part testimonial and part political rally as Tipper Gore assembled her husband, President Clinton and more than 500 advocates yesterday to contemplate her favorite issue: How to improve treatment and understanding of the mentally ill.

The daylong conference, the first major forum on mental health convened by the White House, was designed to expose persistent myths and prejudices that surround Americans with psychiatric disorders--and that prevent them from receiving the same insurance coverage available for other ailments.

"This is the last great stigma of the 20th century," Gore told the hundreds of consumers, lobbyists, researchers and politicians invited to the conference. "We need to make sure it ends here and now."

To illustrate the effects of those stigmas and the helpfulness of treatment, the president, the vice president and their wives shared a stage at Howard University with five Americans ranging from the famous to the obscure, who recounted their first-hand experiences with mental illness.

Television journalist Mike Wallace described his plunge into clinical depression decades ago while he was in the midst of a long libel trial. A Virginia college student told of being so anorexic that she used to allow herself only 20 calories a day. And a native of Hong Kong who was diagnosed with schizophrenia as a teenager in California said he eventually found medication that has enabled him to hold jobs helping other Asian immigrants.

In addition to "putting a face on mental illness very personally," as Tipper Gore said, such testimonials also made another point the White House has been striving to convey. "These are real diseases of a real organ--the brain--and should be treated just like any other medical illnesses," said Steven E. Hyman, director of the National Institute of Mental Health.

Yesterday's conference--chaired by Tipper Gore--was the most visible platform the administration has given the topic. The event also bore political overtones, as Vice President Gore moves to define the issues of his candidacy for next year's presidential election.

For its main session, the conference employed the folksiness and precise choreography that Clinton perfected in his own campaigns--and that Gore has tried to adopt. The president, vice president and their wives sat on stage in small groupings of upholstered chairs separated by coffee tables. And they chatted with a panel that included mental health experts, along with those invited to share their personal stories.

"I know there are some business executives . . . who are still kind of manning the barricades and fighting against opening up treatment for the mentally ill," Vice President Gore said to Wayne Burton, medical director for Bank One Corp. of Chicago.

Burton said his bank had discovered that paying for mental health treatment is cost-effective--an argument that the administration has been advancing for years, but that the insurance industry and business groups oppose.

"It is high time our health plans treat all Americans equally," Clinton told the audience.

In essence, the president and several members of his Cabinet yesterday acknowledged that the federal government's last attempt to require better coverage for psychiatric treatment has proved only marginally successful. Under health reform legislation adopted by Congress three years ago, health plans may not set lifetime caps on mental health coverage any lower than those for other treatment. But they still may restrict the number of doctors' visits for such services and can charge more for them.

Clinton reiterated several announcements about mental health policy that the White House has made over the last few days, including a $10 million experiment to give 1,000 mentally ill Americans who qualify for disability payments coverage for prescription drugs and therapy for which the federal government ordinarily does not pay.

CAPTION: TV journalist Mike Wallace described his bout with depression at a White House conference chaired by Tipper Gore.