While President Carter's plan to restructure the welfare system has gone nowhere in Congress, Sen. Russell B. Long (D-La), chairman of the Senate Finance Committee, has been successfully pushing along a different welfare program - the so-called earned-income tax credit" for the working poor.

Long's plan already is in effect, providing $1.3 billion to more than 6 million beneficiaries a year. And, if the Louisianan has his way on the Senate floor this week, it will grow to almost $3 billion -- nearly half the total now spent in regular federal aid to welfare mothers.

While the program gives billions of dollars to poor families, no one knows whether it is accomplishing the main purpose Long set out for the plan when he first proposed it - to get people "off welfare and encourage them to take jobs."

In addition, critics complain that while the plan may be effective in distributing some money to the poor, it misses many poverty-striken persons who do not file tax returns, it discriminates against taxpayers who do not have children and it raises tax rates for some workers.

As a result, while Long is moving to expand the program rapidly, some analysts are suggesting it is time to step back and review it.

What makes Long's program so unusual is that, unlike previous government efforts to aid the poor, it uses the federal income-tax system either to give tax cuts or to parcel out cash payments to the needy - a first step toward the "negative income tax" so long advocated by conservatives.

Under the law now in effect, workers who do not earn enough to benefit from a tax credit are entitled to claims a direct cash payment from the Internal Revenue Service each April. The IRS mails checks to eligibles as it would mail a refund to more affluent taxpayers.

And in the bill now before the Senate, Long has proposed that low-income workers receive their benefits weekly - directly through the withholding system now used to collect federal income taxes. Employers simply would increase workers' take-home pay to reflect the difference.

If Long's proposals are enacted, some workers actually may see their take-home pay increased to a level that exceeds their gross wages. In effect, the government would be financing a tax-free age-boost.

The credit - Long's brainchild from the start - was enacted in 1975 as "work bonus" for low-income workers with dependent children. The credit is not available to single workers or couples without children.

Long tried three times previously to get the plan enacted, as an incentive to entice welfare recipients to take jobs, but was blocked by the House. The 1975 effort succeeded after the bill was portrayed as a way to offset rising Social Security taxes for the poor.

Under the version now in effect working parents are entitled to reduce their taxes by up to 10 percent of the first $4,000 they earn in wages - for a maximum credit of $400 - with the benefits dropping gradually above that level and disappearing when a worker earns $8,000 or more.

The House voted to extend the present program. But the Finance Committee agreed to expand it, as part of Long's effort to make the tax bill more acceptable to the Carter administration by skewing more of the benefits toward lower-income individuals.

The new version would allow a maximum $600 credit - 12 percent of the first $5,000 in wages - with reduced benefits available for persons earning as much as $11,000 a year. The credit and any direct payments to the extremely poor would be made through the whithholding system.

The difficulty is, the program has some problems, and it is growing so rapidly that planners hardly have time to solve them.

There are these considerations:

While there's no question the money is doing a lot to aid poor families, there appears to be no reliable evidence of whether it is encouraging welfare recipients to take jobs - the major goal Long has put forward as the reason for the program's existence.

John Palmer, a Brookings Institution welfare expert, said analysts simply "don't know how to answer" that question. "It helps make benefits for the working poor more equitable," Palmer said, "we'd be hard - pressed to say it's made a difference" in the welfare rolls.

Tax experts say, despite the government's best efforts, the plan still "misses" large numbers of eligible persons who do not claim the credit or file income-tax returns. Although the IRS has moved to publicize the credit, many low-income workers avoid tax returns altogether.

Critics complain the plan discriminates against low-income persons who do not have children, who may merit extra aid just as much as working parents. "There really isn't any excuse for it," said Rudolph G. Penner, tax analyst for the American Enterprise Institute.

The system for applying for the credit, or the comparable direct cash payment, is so complex it takes considerable clerical ability to compute it - a skill some low-income workers do not have. The House bill moves somewhat to simplify the procedure.

In a few income brackets, mostly around the $7,500 range, the earned income credit actually results in a higher marginal tax rate for workers (that is, the rate they pay on the top dollar of income they earn) - blunting some of the incentive to work toward a higher-paying job.

Moreover, tax experts say the problem can't be corrected easily. The difficulty stems from a quirk in the structure of the tax credit. Long's proposal to make the credit available to higher-income workers would only increase the number of taxpayers in that bind.

Meanwhile, Long's credit remains little known to the general public. A Roper Organization poll published last summer concluded that voters generally do "not understand how the earned income credit works" and would "not favor the concept" once it's explained.