The Senate Finance Committee has quietly taken up a medical-care proposal that is regarded variously as a sham and as the likely next step toward national health insurance.

The committee proposal is not as ambitious as the "comprehensive" national health insurance plan favored by presidential candidate Edward M. Kennedy, among others. In its present form it is also less generous than President Carter's proposed first installment on a comprehensive plan.

Even so, the proposal is huge.

It would for the first time set a limit of $3,500 on the amount that any working family could be called on to pay in medical bills in a year. Any bills beyond that would be paid by a new form of compulsory "catastrophic" health insurance. In addition, it would greatly enlarge Medicaid to extend coverage to millions of mostly non-working poor people who lack it today.

The Finance Committee has already completed the "easy" part, the catastrophic insurance that would go mostly to the middle class. Basically, this would require every employer to provide all employes with a medical insurance policy covering all family medical costs over $3,500 a year. This would be in addition to whatever existing insurance employes have. The premiums would be paid three-fourths by the employer, one-fourth by the employe. An estimated 155 million people would be assured protection.

The committee is divided, however, over the costlier and politically less popular proposal to increase medical aid to the poor.

Medicaid is already costly, and some regard it as wasteful as well. When Congress created it as a companion to Medicare for the elderly in 1965, sponsoring members said it would have little impact on the budget. Today the program aids 22 million people (one American in 10) at a cost of $22.3 billion a year -- $12.5 billion from the federal government, the rest from the states.

Yet Medicaid still misses most poor people.

States are for the most part free to set their own Medicaid eligibility rules. Some states are generous, proffering Medicaid even to non-welfare families with incomes well over the federal poverty line. Others maintain cutoffs well below the poverty level. The result is a patchwork.

One-third of all Medicaid money is now spent in just the two largest states, California and New York -- both relatively generous. Two-thirds is spent in just 10 states, mostly northern and industrial. Elsewhere, in the South especially, the program is often minimal, and Arizona does not have Medicaid.

About 40 percent of the poor are covered by Medicaid through this system; 60 percent -- some 16.5 million people -- are not. The Finance Committee's problem is what to do about these.

The Carter administration proposed extending coverage to about 10.3 million of them by creating a new national income floor below which every household would be eligible for Medicaid. That floor as proposed would be 55 percent of the poverty line, which is now about $7,200 for a family of four and rises automatically each year with inflation. The administration says this would cost $9.3 billion a year, about a 40 percent increase over present costs.

The Finance staff proposed as a less costly alternative that the committee simply spread an additional $3 billion to $5 billion among the states, to sweeten their programs as they please.

This would take pressure off states, many of which have cut back on Medicaid in recent years. Federal offcials say such state cuts typically occur whenever unemployment increases, as it seems to be now, and state welfare budgets rise alongside. Latest federal estimates show, as one possible reflection of these cuts, that the number of Medicaid recipients nationwide has remained stable over the past three years, even though population has grown.

In Finance, however, liberal Sen. Abraham A. Ribicoff (D-Conn.) and a White House health care expert, Dr. James Mongan, have attacked the staff plan as inadequate.

The committee may be moving toward some middle ground which could be reasonably close to the 55-percent-of-poverty administration plan, but less costly. Committee conservtives don't want to spend $9.3 billion.

In completing work on catastrophic health insurance, the committee made an extreme significant change.

This insurance is designed for persons who work and are basically at earnings too high for Medicaid elegibility. However, about 14 million of the 165 million persons who'd be protected are in families with relatively modest incomes -- below $14,000 a year.

At Ribicoff's suggestion, the committee agreed that a family below $41,000 wouldn't have to pay the entire first $3,500 of annual medical expenses before the catastrophic benefit became available; it would only have a pay a quarter of its annual income. Thus if income were $10,000, the family would pay only the first $2,500 of medical bills.

In addition to its general Medicaid proposal, the administration has been pushing a Child Health Assurance Program (CHAP) to extend Medicaid to 2 million more children and 100,000 low-income pregnant women.

New York the other day, before the American Public Health Association, Dr. C. Arden Miller of the University of North Carolina accused Sen. Russell B. Long (D-La.) -- powerful chairman of the Finance Committee -- of "holding CHAP hostages" by withholding his committee's approval until the president agrees to accept a substantial cutback in the $9.3 billion Medicaid expansion.

The committee is far from finished with the proposed legislation. But if Chairman Long and conservative Republicans who don't want to spend $9.3 billion on the Medicaid expansion could strike some bargain with Ribicoff and the president -- not too measly an expansion but not too costly either -- they conceivably could wrap it up by the end of the session and put it on the floor next year.

Long, eyeing the political attractiveness of catastrophic insurance to hardpressed middle-class voters, has said he wants a bill he can run for reelection on, and he thinks Bob Dole (R-Kan.) and Herman Talmage (D-Ga.) want it too.