Sen. Lloyd M. Bentsen (D-Tex.) was an avowed free-trader until about a year ago, when he was struck by the nation's bulging trade deficit and the way it was squeezing American business.

Now Bentsen is a born-again protectionist, a cosponsor of legislation that would put 25 percent tariff increases on exports of countries running large trade surpluses with the United States.

"I've had top executives come to me and say we've got to maintain our markets here and in the world, even if it means moving our plants overseas," Bentsen said in an interview yesterday. He said he sees America's trade problems "just exploding and the administration's doing nothing about it."

The fear that converted Bensten -- who said he long favored free trade as beneficial to his state's agricultural economy -- is widespread in Congress, where protectionist fervor is stronger than it's been since the 1930s. More than 200 bills have been introduced to limit imports and promote exports, moving Congress toward a confrontation with the administration over trade policy.

Aides on Capitol Hill reported yesterday, for example, that members were angry at reports that President Reagan may not provide import relief for the U.S. shoe industry. If he doesn't, Congress could act to impose import quotas, aides said.

The International Trade Commission ruled in June that U.S. shoemakers were being hurt by imports and recommended that the president impose quotas. White House officials have said President Reagan is against import relief for the industry.

Some congressional aides said the president's refusal to grant some relief to the shoe industry would signal that the administration was not serious when it said it might formulate a tough trade policy, and that such response could trigger strong measures by Congress to help not only shoemakers but also other manufacturers suffering from foreign competition.

Protectionist sentiments in Congress are being fed by record trade deficits, which are expected to reach $150 billion this year; the loss of manufacturing jobs and sluggish economic growth that has accompanied the surge, and the feeling that the Reagan administration is doing nothing about all of this.

Senate Majority Leader Robert J. Dole (R-Kan.), traveling in Japan, Taiwan, Hong Kong and South Korea -- some of America's toughest trading partners -- said last week that Congress will act by Oct. 15 on some sort of trade legislation. Trade, he said, will be one of the big election issues in 1986 and 1988.

The 200 pieces of trade legislation that have been introduced in Congress would place restrictions on imports ranging from lumber, textiles, steel, shoes, automobiles and copper to neckties, martial arts uniforms and waterbed mattresses. Only a handful are given a serious chance of passage, however; many of the others are believed to have been crafted more to enhance the sponsors' reelection chances than to help the economy. The bills getting the most attention would:

*Require Japan, Brazil, Taiwan and South Korea to reduce their trade surpluses with the United States by 5 percent or face a 25 percent increase in tariffs on all of their exports. The bill was introduced by Bentsen and Reps. Dan Rostenkowski, (D-Ill.) and Richard Gephardt (D-Mo.). Rostenkowski, chairman of the House Ways and Means Committee, has said he plans to make sure the bill gets action.

*Require the president to offset the increase in Japan's car exports from last year's levels by persuading the Japanese to buy more U.S. goods of the same value or by restricting Japanese imports. This legislation, introduced by Sen. John C. Danforth (R-Mo.), chairman of the Senate Finance international trade subcommittee, is awaiting action by the Senate. A similar nonbinding resolution passed the Senate last spring by 92 to 0.

*Reduce textile and other apparel imports from several countries, especially Korea, Taiwan and Hong Kong. This bill has 60 cosponsors in the Senate and more than 300 in the House.

Allow the president to negotiate telecommunications agreements and require retaliation if trade agreements are violated or foreign trade practices are deemed unfair.

*Authorize countervailing duties against imported articles made from subsidized raw materials. A new version of this bill would include softwood lumber.

*Transfer some trade action authority from the president to the U.S. Trade Representive, increase the president's authority to impose import surcharges and create a new vehicle for relief from foreign industrial targeting.

"I think in theory, it's a great theory," Bentsen said of the free trade principle he long espoused. "But it's not being practiced, and for us to practice free trade in a world where there's much government-directed trade makes as much sense as unilateral disarmament with the Russians."

Economists have said that, in the short run, trade restrictions may help retain U.S. manufacturing jobs, but in the long run they will lead to continued erosion of manufacturing industries, which largely compete with foreign goods, and cause inflation.

One point of contention is how much the economy has been damaged by unfair trading practices and by the strength of the dollar, which has the effect of making U.S. goods more expensive than foreign products. Many economists said that if all trading barriers were dissolved, it would not have much of an effect on trade because the dollar is the major cause of the trade deficit.

Wharton Econometrics studied the effects of a 20 percent surcharge on all imports to suggest the impact of the principal bills under consideration. One of the important aspects of the legislation would be whether it would result in retaliation by other countries, which would further reduce U.S. exports and reduce economic growth.

In the worst-case scenario, Wharton assumed that countries hurt by the tariffs would retaliate; under the best case, they would not. In both cases, the foreign producers would absorb about half of the cost of the tariff so as not to raise prices so high that the producers would lose market share in the United States, according to Nariman Behravesh, chief economist for Wharton.

Under the worst case, assuming that all other conditions are the same, growth in real gross national product -- the inflation-adjusted increase in output of goods and services -- would decline by about 1 percentage point because foreign countries would retaliate against U.S. goods and reduce production here.

Inflation in the first year of the tariff would increase by about 1 1/2 percentage points. The federal budget deficit would fall in the first year by about $45 billion, but then would rise by about $15 billion in each of the next two years, Behravesh said.

The deficits would fall initially because government revenue would increase directly from the amount of the tariff. It would begin rising because production would increase as countries retaliated against U.S. goods.

The trade deficit -- the difference between exports and imports -- would improve by about $25 billion in the first year, $15 billion in the second year and $7 billion in the third year, Behravesh said.

If countries do not retaliate, GNP growth would increase between 0.3 and 0.6 percentage points in the second and third years after imposition of the tariff. Inflation would rise 1 1/2 percentage points in the first year.

The federal budget deficits would decline by $60 billion in the first year and by another $16 billion by the third year without retaliation, Behravesh said. The trade deficit would improve by $38 billion each year for about the next five years, he said.