President Reagan yesterday urged leaders of free-trade lobbying groups to fight protectionist pressures in Congress, arguing that his policies already have begun to roll back the record trade deficits of the past four years.

"The realities of world trade have already begun to shift in our favor, and later this year we'll see our trade deficit begin to shrink," Reagan said in a speech to the faithful gathered by the White House in the Old Executive Office Building. The audience consisted of 135 people, mostly Washington representatives of industries involved in either importing goods or selling U.S. products overseas.

Reagan cited as "the most important aspect of our trade policy" moves by Treasury Secretary James A. Baker III last September to lower the value of the dollar against the currencies of the nation's major trading partners. The Baker initiative was a major policy shift for the administration. As recently as last year, Reagan praised the high dollar as a sign of America's strength.

"Later this year, as order books begin to reflect the new exchange-rate relationships, our exports should begin to grow relative to our imports, a good start in bringing the American trade deficit into balance," Reagan said yesterday.

The president's upbeat tone reflects projections of government and private economists, even though the trade deficit has remained stubbornly high despite a drop of about 23 percent in the value of the dollar against a basket of currencies. The drop in the value of the dollar has been about 30 percent against the Japanese yen and somewhat less against the West German mark.

The lower dollar should improve U.S. sales overseas and make foreign goods more expensive here, but until now importers appear to have been willing to take lower profits to retain their share of U.S. markets. Moreover, currencies of nations such as Canada and the Pacific Rim countries of Taiwan and South Korea are pegged to the dollar and have not fallen. These countries are responsible for 40 percent of the U.S. trade deficit.

The trade deficit currently is running at an annual rate that would be higher than last year's record $148.5 billion, and has acted as a drag on the U.S. economy. As a result, the Reagan administration has lowered its estimates of 1986 growth from 4 percent to about 3.5 percent.

Commerce Secretary Malcolm Baldrige, as well as economists from Data Resources Inc. and Wharton Econometrics, said they expect a turnaround in the trade deficit later this year. Baldrige has said the deficit likely will end up in the $150 billion range.

Any turnaround is likely to be too late to stop legislation now before Congress described yesterday by Reagan as "destructionist."

Yesterday's meeting with Reagan marked a new effort by the White House to keep the Senate from passing legislation similar to that already approved by the House. In opposing the legislation, however, Reagan appeared to hold out an olive branch to "thoughtful members of Congress" who favor "pro-growth" trade policies the administration supports, such as authority for a new round of global trade talks, a $300 million war chest "to combat unfair foreign subsidies," revisions of antitrust laws to make U.S. industries internationally competitive, and greater protection against piracy of trademarks and patents.

Other members of his administration -- including Baldrige, White House Chief of Staff Donald T. Regan and U.S. Trade Representative Clayton Yeutter -- were reported as telling the free-trade lobbyists that the government is willing to work with Congress on a "responsible" trade bill. Many Senate Republicans and Democrats have accused the administration of refusing to work with Congress on trade legislation.

In his speech, Reagan called on the free-trade groups "to stand with us again in our dealings with the Congress in this matter. And please join us in urging the Congress to stop shirking and get back to its real work -- not fostering economic timidity, but promoting economic incentives, innovation and growth."

Reagan provided the ammunition with a positive appraisal of his trade policy, including a more aggressive attack than any other administration on other countries' unfair trade practices. He cited positive results from trade complaints against Japan and the European Community, and warned that Japan would face retaliation if it fails to agree to open its market to U.S. semiconductors and to stop dumping its products here at less than their fair value.

He revealed that preparations for a new round of global trade talks should be completed this week, despite opposition from some developing nations, leading the way to the start of talks in September. These talks, Reagan said, will help remove barriers to trade in sectors such as agriculture and services that are important to the American economy.