The House Ways and Means Committee yesterday approved a scaled-down but still dramatic reconstitution of the nation's major welfare program, which now supports nearly one in 20 Americans. The vote was 21 to 14.

The bill, requested by President Carter after an even more ambitious welfare plan failed in Congress last year, for the first time would set a minimum level for state welfare payments under the program of aid to families with dependent children.

Under the bill, once the program goes fully into effect in 1982, the states would be required to support needy families at a level of benefits equal to at least 65 percent of the poverty line.

This works out to a benefit payment of $4,700 a year in 1979 dollars for a family of four without any other income. At present 13 states, mainly in the South, pay less than this.One, Mississippi, pays $1,200 in cash support benefits. The $4,700 guarantee would consist of cash payments plus food stamps.

About 800,000 families would get higher benefits under the provision.

By pegging the minimum payment to 65 percent of the proverty line, the bill would also provide a partial automatic cost-of-living adjustment for the welfare beneficiaries. The poverty line is recomputed every year and has been rising rapidly with inflation. Each time it rises, the dollar level needed to meet the 65 percent minimum would rise, too.

Another key feature of the bill would impose uniform federal rules on income and assets to determine who is eligible for welfare. This, together with federal assumption of a higher percentage of overall program costs, would go a long way toward federalizing the program -- a point cited in criticism by Republicans who voted uniformly against the bill yesterday.

Under the new assets test in the bill, a person's house and lot, regardless of value, his clothes and furnishings, an auto used in work and an added private auto, wouldn't be counted in determining whether a person without other income has enough assets to support himself. The bill provides that if after these deductions he has liquid assets exceeding certain specified amounts -- to be set by the state at no lower than $750 or higher than $1,750 -- he can't get welfare.

The effect of all these changes is to make the program -- which now has 10 million beneficiaries and costs the federal government and the states $12.4 billion a year -- far more uniform nationwide.

The federal government already reimburses the states from 50 percent to 78 percent of each state's outlays for the dependent children program. Under the new financial formulas in the bill, the federal share would rise by 5 percentage points.

This additional federal matching means that the states would have to spend hundreds of millions less of their own money, while at the same time complying with ever tighter federal standards.

The bill approved by Ways and Means yesterday, with all 21 votes in favor coming from Democrats, is half of a two-part package constituting President Carter's welfare program. The other part, which is being handled by the Labor Committee, would provide 618,000 public service jobs for welfare clients to help them get off the welfare rolls and get job experience, at an average annual salary of $7,200 a job.

The two parts of the package combined would raise the costs of the existing welfare programs by about $5.5 billion a year if both bills clear Congress. They would give the states about $900 million in fiscal relief through federal assumption of the higher reimbursement percentages and job costs.

The 13 states whose combined welfare and food stamp benefits for a family of four currently fall below $4,700 are: Mississippi, Alabama, Louisiana, Florida, Georgia, Texas, South Carolina, Tennessee, Arkansas, Kentucky, New Mexico, North Carolina, and Arizona.

The bill's sponsor, Rep. James C. Corman (D-Calif.), and Ways and Means Chairman Al Ullman (D-Ore.) as well as administration officials on the scene expressed delight after the 21-to-14 vote.

"I am very pleased," said Corman. "This is a substantial improvement in the welfare system."

At Corman's suggestion, the committee agreed to seek arrangements for floor debate that would bar floor amendments and allow only one Republican motion to recommit the bill.

The bill still faces formidable obstacles, however, including almost solid GOP opposition. Several Republicans offered different versions of state bloc-grant amendments -- allowing states to experiment on a large scale with different welfare systems including programs to force people to work off benefits -- but all were beaten back by party-line Democratic majorities.

John H. Rousselot (R-Calif.), Willis D. Gradison Jr. (R-Ohio) and others said these amendments would move away from increased federalization and also help cap costs in some cases, instead of increasing them.

"I think we can mount a substantial attack on several specific points on the floor," said Gradison, citing the "costly" nature of the bill and the fact that it would let people with expensive houses, no matter how valuable, get welfare if they don't have current income. "The price-tag is high; it's a giant step toward increased federal control," he said.

And in the Senate, Finance Committee Chairman Russell B. Long (D-La.) has called the bill expensive, and a disincentive for welfare clients to work and get off welfare.

Welfare lobbyists, jubilant after the vote, said they realize Long is powerful but vowed to work to beat him.