Walter (Fritz) Mondale appears to have had some second thoughts on his strident demand that this country "get tough" with Japan so as to recapture lost jobs and markets.

The former vice president's speech to the steelworkers union on Sept. 22 (" . . . the key task . . . is to stop flying that white flag, to start running up the American flag . . . ") earned him a protectionist label, out of keeping with his long-standing support of free trade.

Mondale didn't like the label: Friendly columnists were persuaded that he was misunderstood, or even misrepresented. But Mondale's words -- both his speech, and a defense of it in a communication to The Washington Post -- were crystal clear, and could not have been misunderstood.

Among other things, he backs "local content" auto legislation, which in effect would require popular Japanese cars to be mostly made with American materials and labor. President Reagan's trade ambassador, Bill Brock, correctly calls this highly protectionist bill, which has now passed the House, the worst piece of trade legislation since the Smoot-Hawley tariff bill that helped trigger the Great Depression.

But I think Mondale is beginning to realize that his Sept. 22 speech risked alienating a segment of his liberal constituency. His uncharacteristic harangue represented, in my view, an effort to qualify for the AFL-CIO endorsement for the Democratic presidential nomination.

A more restrained approach makes sense, because with Teddy Kennedy now out of the race for the Democratic nomination, Mondale is the front runner: he shouldn't have a bitter fight on his hands for labor approval.

Mondale also heard from economists whom he respects -- and political advisers whom he trusts --that his approach on the trade issue was wrong.

I don't want to convey the idea that Mondale has issued a "mea culpa," and abandoned his view that the United States should "get tough" with Japan. He hasn't. But in a presentation the other day to the Council on Foreign Relations, and in a conversation with me in his law office here, the "get tough" talk was muted. It's something like the sixth thing he mentions in a broad six-point program.

The first point on his agenda now is the urgent need for economic growth to reverse stagnation here and abroad. Growth, he says, is the key to everything, because without it, a protectionist struggle to bite off pieces of a shrinking global pie is inevitable. Because the world is so interdependent, the effort must be coordinated -- no nation is strong enough to sustain growth on its own.

You can comb through the Sept. 22 speech, and you won't find the words "global" or "growth" mentioned anywhere.

In taking a new approach, Mondale appears to be relying on the advice of economist C. Fred Bergsten, assistant Treasury secretary in the Carter administration who now heads the Institute for International Economics here -- one of the most outspoken antiprotectionist think tanks in town.

"We've got to stress growth to avoid these problems of Third World debt. And we've got to help them pay off the debt by increasing the resources of the IMF and the World Bank," Mondale told me.

That's an important revision from Sept. 22. In his steelworkers speech, he argued that international institutions and American banks had loaned too much money abroad, instead of "invest ing it in competitive new plant and equipment here in the United States . . . "

Mondale endorses the Bergsten theory that a basic reason for world economic instability is the misalignment of exchange rates, which he blames mostly on Reaganomics: by shifting from Reagan's tight-money, easy-fiscal mix, Mondale thinks worldwide growth would be stimulated, easing protectionist pressures.

The former vice president then ticks off a series of other problems, including European agricultural subsidies, a shortage in Export-Import Bank funds, the scuttling of trade adjustment assistance by the Reagan administration. And he wants to stimulate productivity by "a new labor-management strategy."

Only then does he come to the "get tough" proposals. He sticks to his belief that the Japanese won't be persuaded to open their markets more fully unless they know that the United States means business.

He argues that unless Japan is convinced of the need to take some highly visible steps that will reduce their boxcar-sized bilateral trade surpluses with the United States, the congressional demands for massive protectionist steps will be overwhelming.

But Mondale appears to recognize that the problems run deeper, that some basic changes are needed in labor-management relations, especially in the declining U.S. industries. He concedes that blocking Japanese cars by quotas or the local content approach won't solve the problems of the American auto industry.

Increasingly, Mondale seems to be coming around to the view that the noncommunist world faces a common economic problem that needs a common -- not a beggar-thy-neighbor -- approach. That's more sensible than the idea that both he and Sen. Robert J. Dole have been tempted by -- the notion that a focused, or limited protectionist zap against the Japanese can avert a bigger protectionist war.

To paraphrase the late Leon Henderson of World War II price-control fame: Being a little bit protectionist is like being a little bit pregnant.