The Democrats' new issue?

I thought Walter Mondale maintained his poise in an exchange with a panel of reporters on ''Meet the Press'' this week. For Europeans who don't understand, the United States is now going through the preliminaries of picking presidential candidates for 1984. Long before then, I fear, we shall go through a boring interval; however, excitement will pick up later.

I have a special feeling for Mr. Mondale, I suppose, because his son used to help my wife arrange vegetables at the out counter at a local supermarket where he had a part time job. (I hope my personal relationship has not biased my political judgment.) In any case, this week Mr. Mondale (that is, the former vice-president) had a role to play: to present a presidential image on television, show his knowledge of issues, and avoid criticizing other Democratic presidential hopefuls lining up for the race. I thought he managed the delicate affair presentably.

A new element has entered the matter recently, however. It was underlined the other day by former Sen. Edmund Muskie in a speech at the Coast Guard Academy. He asserted a change of direction by the administration from domestic to outside problems:

''We have seen a shift in US policy from an emphasis on internal reform and indigenous factors to an emphasis on the Soviet and Cuban role,'' he said. ''We cannot ignore this role, but are we allowing anticommunist paranoia to skew our vision?''

Quite obviously, President Reagan can respond that he must react to world crises as they occur, but Mr. Muskie, first, and then Mr. Mondale expressed uneasiness over the way things are going. Said Mr. Muskie: ''I have not yet heard adequate explanation and justification and I am not persuaded.''

The group of reporters that interviewed Mr. Mondale June 19 were looking for a story. It was rather amusing to see how their guest dodged and ducked. He kept command of the situation, however, even to filibustering his replies. But the fresh emphasis was about Central America and US response. Is Mr. Reagan introducing a new political issue in the anxious capital? Mr. Mondale wonders. He first tried to define the problem. Yes, he agreed, if the Soviets or Cubans establish a major base or military installation in Central America it will be a matter of ''very, very severe concern.'' It will affect our ''vital interests,'' he said - the code phrase that means US counteraction.

But that's hypothetical, Mr. Mondale said. So far, he said, we have followed a good neighbor policy - ''a policy that was principally based on political and social reforms.'' He spelled these out as he saw them (while the impatient TV panel fidgeted); he argued they were showing moderate success. Now, he said, things are changing under administration pressure:

''There was a problem in Central America but it wasn't a crisis. They (the Reagan administration) ended it and turned it into principally a military venture with very little, if any, emphasis on political reform or on efforts to bring about a negotiated settlement.''

Mr. Mondale seemed to have carefully rehearsed this argument and was seizing the opportunity for attack. While everyone remembers Vietnam, Mr. Mondale is questioning Central America. The administration, he charged, is ''going full speed ahead for a military solution. And I believe, based on what I see, it's almost inevitable that American troops will be sent down there.''

These are scare words in a political contest addressed when Americans are particularly sensitive. It wasn't like this in 1898: The Pacific Squadron of our smart little navy steamed into Manila Bay and without losing a man junked the Spanish fleet. In Cuba the doomed Spanish fleet steamed out of Santiago Bay and was destroyed in a few hours running flight. Exit the Spanish Empire.

Now things are different. The martial trumpet blows a different note. At a speech in Jackson, Miss., June 20, Mr. Reagan tries to rouse the country. He calls on America to reject ''those who would disarm our friends and allow Central America to be turned into a string of anti-American Marxist dictatorships.''

Is he referring to Mr. Mondale? Mr. Reagan sees ''no excuse'' for not providing Central American friends with ''the weapons they need to defend themselves.'' Failing this he warns of a ''tidal wave of refugees . . . seeking a safe haven from communist repression to our south.''

Here is a presidential issue a long way before election.