U.S. Trade Representative William E. Brock Jr., attacking growing moves toward protectionism as harmful to economic recovery, yesterday challenged former vice president Walter F. Mondale to a debate over the Democratic frontrunner's speeches advocating a "tough" trade policy to save American jobs from Japanese imports.

"Protectionism is an indictment of us and will do more damage to the United States than any other country," Brock said in a National Press Club appearance.

He said he "would love" to debate Mondale "next year or right now" on the issue of protectionism and what it means for the continued vitality of the American economy.

Mondale, a traditional free trader, caused a stir last fall with a series of speeches to labor organizations that accused other countries--especially Japan--of not playing fair in trade. He pushed in strong, colorful language for "a national policy that fights back and allows American workers their fair shake in the competitive markets of the world."

These speeches, Brock said, have caused concern in other countries whose leaders see the United States as becoming increasingly inclined toward protectionism--a move he pledged the Reagan administration will continue to fight.

"This president and this administration are committed to free trade and we are going to take every possible step in that direction" despite occasional steps backward, he said.

The less-developed countries of the world--still suffering from the global recession and hardest hit by past oil price rises--buy more American-made products than Europe and Japan together, Brock said. But the United States also imports half of all goods manufactured by Third World nations.

"Their well-being is important to us," Brock asserted. "They must have an opportunity to earn foreign exchange from exports. That simply cannot be done if protectionism takes root in advanced nations, as so many now advocate.