They came in vans from the Prince George's County suburbs--150 or so senior citizens invited to hear the congressional perspective on health issues and the news they got was this: Ronald Reagan, a senior citizen himself, is a threat to their health care.

Admittedly, the perspective had a partisan twist to it: The host of the event was Prince George's Democratic Rep. Steny H. Hoyer and the main speaker was his Democratic colleague from Baltimore and Howard County, Rep. Barbara A. Mikulski.

But the slaps at President Reagan and the Republican administration's efforts to curtail Medicare expenses by altering benefits drew the loudest applause from the white-haired Marylanders.

"The good news is we do have Medicare in this country," Mikulski said. "The bad news is it doesn't provide enough. We're working to change that. I feel the way to do that is to get a new president, a Democratic president."

"Absolutely, absolutely," said one woman at the back of the hearing room, where a table of punch and cookies had been set up. Later, the woman, identifying herself only as a former federal employe, told Hoyer, "We are being mounted and stuffed with the cost of medical care."

The Reagan proposal, which the administration submitted as part of its fiscal 1984 budget, would provide new "catastrophic" benefits for those hospitalized for more than 60 days. But it would increase costs for Medicare patients who spend less than two months in hospitals. Most Medicare beneficiaries fall into this latter category.

Medicare provides health benefits to about 29 million aged and disabled Social Security recipients. It costs more than $55 billion a year and, according to congressional and administration officials, is threatened with bankruptcy by 1991 if some action is not taken. The administration's proposal, which is stalled in Congress, could save $500 million or more a year, according to some estimates.

The seniors were told by congressional committee staff members that the Reagan administration also has been cutting the Medicaid program, which provides health benefits for poor people of all ages. Medicaid funds to all states have been cut and will be reduced again next year, forcing cuts in services to the poor.

Maryland, for instance, lost $5 million in federal Medicaid money during fiscal 1983, which ends June 30, and will lose another $12 million in fiscal 1984.

At the end of the Hoyer-sponsored "seminar" for the hometown folks, Louise Kleinheitz of Mount Rainier analyzed the state of senior health care this way: "I guess the best thing is to just not get sick."