The chairmen of the House and Senate tax-writing committees proposed yesterday that Congress enact a value-added tax, a complex form of national sales tax, to replace part of the Social Security tax and finance a spate of other tax reductions.

Chairman Al Ullman (D-Ore.) of the House Ways and Means Committee and Chairman Russell B. Long (D-La.) of the Senate Finance Committee both suggested a 10 percent tax that Ullman estimated would bring in between $120 billion and $150 billion a year.

The two legislators stressed that the tax they envision would not increase the overall tax burden.Both said they saw the levy as a "replacement tax" to enable the government to roll back Social Security taxes and reduce income tax rates.

Ullman conceded that the value-added tax ultimately would be passed on to consumers, and as a result would boost prices. However, he said lawmakers might exempt food from the tax, providing some relief for the poor.

The Ways and Means Committee chairman said his panel will begin hearings on the plan next month, in hopes of having a measure ready for consideration by Congress next year. Both Ullman and Long predicted that some form of value-added tax would be enacted in any case in 1981.

Enactment of a value-added tax would constitute a major turnabout in the nation's tax policy. The income tax has been the traditional mainstay of the federal tax system. The primary exception has been the payroll tax that finances the Social Security system.

Replacing parts of the income and the payroll tax with a value-added tax would shift the burden of the federal tax system away from income and place it on consumption. The value-added tax has been used widely in Europe. Britain recently increased its value-added tax to finance a cut in income taxes.

The value-added tax has been criticized by liberals as regressive, in that it hits the poor proportionally harder than it hits those in higher income brackets. By contrast, the federal income tax generally scales the tax bur- den to the ability of the taxpayer to pay.

In recent years, however, conservatives would be helpful in encouraging more investment and savings and also would help spur exports. Under current international trade rules, a value added tax can be rebated to exporters, while the income tax cannot.

A value-added tax is a complex form of sales that is levied at each stage of the manufacturing process. Each producer or seller pays a tax based on the amount by which he has increased a product's value. Ultimately, the entire tax is passed on to the consumer.

The only significant use of a value-added tax in this nation has been in Michigan, which enacted one several years ago to replace corporate taxes. However, the complex corporate taxe is not expected to serve as a model.

Although Long and Ullman mentioned only the investment and export incentives yesterday, they previously have indicated that the main attraction of the value-added tax is their belief that there is no politically practical alternative.

Ullman, in particular, has complained that the income tax burden now is so high that the lawmakers cannot realistically boost taxes to pay for new spending obligations. And Congress already is under pressure to roll back scheduled Social Security tax hikes.

Although the two lawmakers apparently did not consult each other before outlining their plans yesterday, they appeared at the same forum -- a symposium held by the U.S. Chamber of Commerce -- and they produced similar lists of tax reductions that enactment of a value-added tax would be designed to offset. The proposals included:

A major rollback in Social Security payroll taxes, which are scheduled to rise sharply in 1981. Although Long gave no precise figures, Ullman suggested a 25 percent reduction in taxes paid by employers and by workers.

At least some cuts in federal income tax rates to ease the burden on low- and middle-income taxpayers and to slash the maximum tax rate on investment income to 50 percent from the present 70 percent.

Faster depreciation writeoffs for business to stimulate more investment in new plants and equipment, a step many economists say is needed to help increase the nation's lagging productivity.

Elimination of the present "double taxation" of corporate earnings, first as company income, then as stockholders' dividends.

Ullman also proposed using the value-added tax to help finance new incentives for saving, possibly by allowing savers to defer payment of taxes on the interest earned on money deposited in savings and loan institutions.

The Ullman-Long proposal was not welcomed universally at the symposium yesterday. Rep. Barber B. Conable (R-N.Y.), ranking minority member of the Ways and Means Committee, delivered a lengthy oration opposing the measure, warning his audience not to "rush to embrace it."

Among Conable's criticisms were that the tax would be inflationary and most likely regressive, that it would be difficult to administer and that it might dampen demand for goods. He also raised the possibility that it would be used to increase the nation's total tax burden.

Although the support of Ullman and Long provides important backing for the new proposal, both lawmakers conceded it was far from a shoo-in, at least under present conditions. The measure is opposed by most liberals and by most major labor organizations.