Last year, devout free-trader Sen. John C. Danforth (R-Mo.) preached that government should stay out of the business of protecting American industry against foreign competition. "The business of America is business," Danforth said.

Now after a year in the pulpit as chairman of the Senate finance international trade subcommittee, Danforth's sermons contain fire and brimstone against Japanese traders and warnings that the almighty Congress may unleash its wrath on Japan in the form of reciprocity legislation if it doesn't effectively open its markets to foreign goods.

Before he may have been a disciple of free trade, but now Danforth is the leader of the reciprocity movement, a new religion on Capitol Hill sweeping up converts every day. The movement's message is: Do unto other countries in trade what they do to the United States.

Danforth has seen the political light. "This is a maturing," Danforth said. "Right now my view is this country has got to rebuild its economic strength in the years immediately ahead and that cannot be accomplished" if foreign countries shut out American goods.

Political "maturing" is not new to Capitol Hill where ideals change when confronted with reelections, constituent pressures and other realities. But Danforth denied that his metamorphosis was based on the facts that the faltering auto industry is important to his state and that he is up for reelection in the fall.

"This isn't a spurt of activity that will decline because of the reelection," said Danforth, who so far has no serious opposition. His maturation is the result of growing in his job as trade subcommittee chairman, he said.

Last year Danforth, who said he reluctantly led the fight in Congress to place import quotas on Japanese car imports, believed "international trade is a commercial arrangement between" businesses, not governments. "That is too Coolidgesque," Danforth said, referring to the president known for his restrictive trade policies. "I'd like to retract that."

"Before all we did was complain," Danforth said. "We traipse over to Japan, and we bellyache. If we complain loud enough and whine and plead and threaten . . . perhaps the Japanese will change. That has limited effectiveness, and it's demeaning."

"The notion of reciprocity and trying to put together a bill is an idea that has evolved in the last few months as it has become clear rhetoric is not effective and creates ill will," Danforth said.

However, he added, "It's possible to create a really protectionist bill in the name of reciprocity, and we have to watch that very carefully."

While Danforth now feels some troubled industries may need temporary import relief, he said he doesn't believe in the government trying to "breathe life into a corpse." His new belief favoring "limits on imports from other countries for a short time is clearly a maturing," he said.

Reciprocity in trade is expected by some administration trade officials to be the hottest topic on Capitol Hill after the budget is considered. Already, Danforth has tentatively set hearings on reciprocity legislation for March 25.

Administration officials have endorsed the general concept, particularly toward Japan, which is projected to have a $20 billion trade surplus with the United States this year. And increased protectionist sentiment on Capitol Hill, particularly in light of increasing unemployment in import-sensitive industries, will assure easy passage of some type of legislation, Danforth said.

Danforth first floated the idea of reciprocity during hearings last December on Japanese automobile imports. Congress' ferver was fueled by the deepening troubles facing the American auto industry while Japanese cars are still sold here, although in slightly fewer numbers, and the trade deficit with Japan, which is expected to reach $20 billion this year.

In the past, Danforth has espoused a free-trade policy allowing open access by foreign countries to U.S. markets. But this legislation would in some cases allow the president to prevent foreign firms from selling or investing here if their markets aren't open to U.S. companies.

Danforth introduced his own bipartisan-sponsored legislation last week that would "establish reciprocal market access as a principle of U.S. trade policy," he said. His legislation would require the administration to identify barriers to U.S. exports and submit to Congress trade barriers of other countries and their impact on American exports.

In addition to Danforth, Sens. John Heinz (R-Pa.), John H. Chaffee (R- R.I.), William V. Roth (R-Del.), Daniel K. Inouye (D-Hawaii) and Robert W. Kasten (R-Wis.) have all sponsored reciprocity legislation so far, all of which will go through Danforth's committee. And some House members have also introduced bills.

Under Danforth's legislation the president would also have to proposed actions to redress imbalances in trade access. In addition, the legislation toughens existing law, allowing the president to act against unfair trading practices of other countries. The law under Danforth's proposal would allow the president to act against countries that have a lack of "substantially equivalent commercial opportunities" for American firms. Under existing law, the president can withdraw earlier trade concessions, impose duties, fees and other restrictions if he finds unfair trading practices. Danforth's amendment would also allow him to restrict investment in the United States by an offending country.

The president would have explicit authority to redress offending trade practices.

Some of the proposals would give the president the power to enter into negotiations to redress grievances, tell the U.S. directors of the International Monetary Fund and the World Bank to vote against loans to offending countries and provide legislation providing barriers identical to those in other countries and adjust U.S. government procurement policies.

"Reciprocity does not mean exports must equal imports," Danforth said. "It means all sides to a trade relationship have a fair opportunity to market what they produce."