Leading proponents of pr otectionist legislation in Congress yesterday expressed skepticism about the effectiveness and sincerity of Japan's new plan to open its markets to U.S. imports and said that they plan to continue to press forward with bills aimed at Japanese imports.

The White House said it would withhold judgment on the Japanese plan, announced Monday, "until the effect of the program on our exports is realized."

Ironically, the government reported yesterday that the U.S. trade deficit with Japan last month equalled a record set in July 1984 and that imports of Japanese automobiles hit an all-time high in June.

The Japanese pledge to lower barriers to their markets "comes at a crucial time in Japan's trading relations with us and with her other trading partners," said White House spokesman Larry Speakes. "While United States relations with Japan are amicable and cooperative in nearly all respects, trade issues have been a source of deep and growing concern.

"While a long-term effort is welcome, earlier implementation would help resolve the crucial trade problems confronting us today," he said.

A senior White House official said the statement was intended to dampen protectionist forces in Congress while keeping pressure on Japan to make substantive changes in its trade policies.

In an attempt to head off growing protectionist sentiment in Congress, the Japanese government approved a three-year market-opening plan that included the reform of 88 regulations that foreign business people have complained about.

Although the main points of the "action program" had been announced earlier, the Japanese government disclosed new details of the plan. In addition, Prime Minister Yasuhiro Nakasone asked the Japanese people to "willingly accept foreign products."

The "action plan" was greeted with skepticism on Capitol Hill, where one major piece of trade legislation against Japan is awaiting action by the Senate this week.

"I haven't read the Japanese proposal, but given past experience I'm skeptical," said Sen. Lloyd Bentsen (D-Tex.), one of the cosponsors of legislation that could lead to a 25 percent additional duty on imports from Japan, Brazil, Taiwan and South Korea. "The Japanese bureaucracy long ago mastered the hidden trade barrier."

That legislation is also cosponsored by House Ways and Means Committee Chairman Dan Rostenkowski (D-Ill.) and Rep. Richard Gephardt (D-Mo.). All three congressmen said the Japanese actions would not prevent them from going forward with their legislation.

Rostenkowski told a National Press Club luncheon that Japan's proposal is "a positive stick the president can use." However, he also expressed skepticism about the plan and said, "until such time as they blink and we receive that as a blink, I'm going to push this bill."

Sen. John Danforth (R-Mo.), chairman of the Senate's international trade subcommittee, said he still planned to go ahead with two major pieces of legislation that call for retaliation against Japan if their markets are not opened to U.S. goods. A Danforth aide said one bill is aimed only at Japan, the other addresses telecommunications products from all trading partners.

"The announcement that Japan will take another package of steps to open markets sounds good," Danforth said. "But we have heard fine-sounding words before and seen very little by way of results. I hope the new announcement amounts to something, but I must say that I am skeptical."

"We have negotiated with great patience for years," Danforth continued. "If the United States is to enjoy any credibility we must take retaliatory steps to help ensure that this package does not prove to be as hollow as its predecessors."

Sen. John Heinz (R-Pa.) noted that "Prime Minister Nakasone has just announced the sixth market opening package in four years. None of the others has done much good and I suspect that the current one will have just about the same effect.

"I would argue that the time is long past for continued discussion and negotiations," Heinz continued. "These endless negotiations seem to have taken on the character of an elaborate con game, with the United States as the willing dupe. The Japanese have no incentive to make meaningful changes in their market structure because they are confident that we will never take any action against them."

Sen. Max Baucus (D-Mont.), another trade subcommittee member, warned that if the Reagan administration doesn't take strong action to open Japanese markets, "even more drastic measures will be taken by Congress in the future."

In an interview yesterday, Japanese Ambassador Mobuo Matsunaga said that the Japanese government understands that "tangible results" have to follow the action program, "or it will not be appreciated." But the diplomat also stressed that success of the program in stimulating imports will depend heavily on efforts made by the United States and Japan's other major trading partners. will have just about the same effect.

"I would argue that the time is long past for continued discussion and negotiations," Heinz continued. "These endless negotiations seem to have taken on the character of an elaborate con game, with the United States as the willing dupe. The Japanese have no incentive to make meaningful changes in their market structure because they are confident that we will never take any action against them."

Sen. Max Baucus (D-Mont.), another trade subcommittee member, warned that if the Reagan administration doesn't take strong action to open Japanese markets, "even more drastic measures will be taken by Congress in the future."

In an interview yesterday, Japanese Ambassador Mobuo Matsunaga said that the Japanese government understands that "tangible results" have to follow the action program, "or it will not be appreciated." But the diplomat also stressed that success of the program in stimulating imports will depend heavily on efforts made by the United States and Japan's other major trading partners.