As a militant free trader, then-Sen. Walter F. Mondale (D-Minn.) led a filibuster in 1970 that ultimately killed a labor-supported bill to restrict textile imports from Japan because he considered it protectionist.

Twelve years later, courting labor's support in his race for the Democratic presidential nomination, former vice president Mondale trumpeted a different line as he painted a picture of American kids "sweeping up around Japanese computers" if the United States did not start "acting tough" on trade matters.

That speech to the United Steelworkers of America convention last fall catapulted trade into the front rank of presidential campaign issues for the first time since 1884, when Grover Cleveland, a Democratic free trader, narrowly defeated Republican James G.

This is one of a series of occasional articles on presidential candidates' positions on issues. Blaine, who ran on a platform of high tariffs.

Other Democratic candidates also have sounded protectionist themes, and President Reagan has continued to speak out for free trade.

Trade facts and figures have become almost a litany on Democratic and Republican hustings: exports account for 5 million U.S. jobs and four of five new manufacturing jobs created between 1977 and 1980, two of five acres planted by American farmers produce crops for overseas markets, and total trade in goods has jumped from 8.3 percent of the gross national product in 1970 to 14.9 percent last year.

In statements submitted to The Washington Post, the six announced Democratic candidates took varied positions on trade policy and ways to reverse last year's record $31.8 billion merchandise trade deficit. Nor did they coalesce on labor-supported domestic-content legislation, which would require certain percentages of U.S.-made parts in cars and trucks sold here and which provided the focus for last year's trade debate in Congress.

The Reagan administration and some Democrats attacked the bill, which passed the House but never reached the Senate floor, as the worst trade bill since the 1930 Smoot-Hawley Act raised tariffs to levels so high they have been blamed for worsening the Great Depression.

Thus, despite support for domestic-content legislation last year from four of the six announced Democratic candidates, only Mondale and Sen. Alan Cranston (Calif.) embraced it fully in their statements to The Post. Sens. Ernest F. Hollings (S.C.) and John Glenn (Ohio), who supported the bill last year, failed to mention it in their statements.

Neither did Sen. Gary Hart (Colo.), who took a more equivocal position last year by telling United Auto Workers President Douglas A. Fraser that he would support it only if it were the only way to save the U.S. auto industry. Without mentioning domestic content, Hart said in his statement that "protectionist solutions" for one industry's problems "can ricochet and produce worse problems in other industries."

Former Florida governor Reubin Askew, President Carter's one-time special trade representative, was the only candidate who took a strong stand opposing the bill last year and in his statement to The Post.

Askew emerged as the freest trader among the candidates, calling for "freer trade on fairer terms" and attacking "misguided adventures into protectionism" while supporting "strictly enforcing" U.S. trade laws against unfair practices.

Mondale attacked Reagan administration policies as harmful to U.S. trade positions while Cranston, Hollings and Glenn approved strong measures against other countries' unfair trade practices.

"The only way to remove a barrier is to raise a barrier," Hollings said. Glenn said the "injurious effects" of other nations' industrial policies "must be neutralized."

Cranston, Glenn and Mondale called for cooperative efforts to correct the imbalance between the strong dollar and other currencies, which raises the cost of U.S. goods overseas.

The rallying of Democratic candidates to the trade issue has provided a vivid example to the GOP of its potency in the coming presidential campaign and, moreover, has exerted an influence over administration trade policies.

U.S. Trade Representative William E. Brock, a former GOP chairman, suggested this winter that the Japanese should grant U.S. car makers two years of restraints on imports instead of one, to keep the issue from coming up amid next year's election.