If President Reagan has a "window of vulnerability" on the war-and-peace issue -- and there is some question that he does -- it is open now.

Presidential bungling has given Democratic challenger Walter F. Mondale a chance of painting the incumbent as an ignorant incompetent on whose desk a sign should read, "The buck stops everywhere but here."

Last week's master script called for Reagan to step forward in the soft garments of the peacemaker.

At the United Nations, he spoke of "love and trust" in dealing with the Soviet Union.

The idea was that Soviet Foreign Minister Andrei A. Gromyko would go home and tell the Kremlin that Reagan is a strong and invincible leader, who commands the admiration of his country and the world.

But the Lebanese terrorist attack on the U.S. Embassy annex and the president's thrashing and crashing about to escape responsibility put a crimp in the plans. Reagan's first alibi -- that the embassy's security preparations were like a protracted kitchen remodeling -- ranks with the schoolboy's "The dog ate my homework" dodge.

A subsequent effort to unload the guilt on former president Jimmy Carter's back proved too much even for that abject loyalist, Vice President Bush. In fact, Reagan succeeded in producing the first wave of sympathy that Carter has enjoyed since leaving the White House. The outcry was so considerable that Reagan had to call on the man he blames for everything and "explain" what he meant.

Murmurs about news media "distortion" did not survive a study of the transcript. Finally, the president fell on Congress as the scapegoat in what he called the decline of our intelligence services. Actually, since the history was clear -- two previous attacks in the past 17 months -- and the terrorists had warned that they would strike again, nobody believed that the new casualties could have been averted simply by more or better intelligence.

Gromyko doubtless followed the squirming with considerable interest. He concluded his call at the White House with an unmistakable, although unstated, assertion that he did not care to become a volunteer in the Reagan-Bush reelection campaign.

Reagan faces another crisis of his own making in dealing with the Nicaraguans. They have crossed him up by calling his bluff: They have agreed to do what he demanded. The Sandinistas plainly have figured out that it is better to deal with him now -- while there is campaign pressure, however minimal, on him -- than later, after he, presumably, has been triumphantly reelected.

They have, in short, said that they will sign the Contadora Treaty, a regional agreement that Reagan often piously endorsed as the way to peace while unleashing the "contra" mercenaries to harass the Nicaraguans into it.

The response of State Department officials to what easily could be interpreted as a most satisfying outcome of their exertions has been livid indignation. They have acted as if the Sandinistas' capitulation to reason were a communist plot. They mutter darkly about a "propaganda coup" and have fallen to squawking about the shape of the table.

Reagan has denied that he will for settle for nothing less than the overthrow of the "Marxist-Leninist" regime in Managua. But now that the Nicaraguans forced the moment of truth on him, his people are in a panic. Our special envoy is trying frantically to rewrite the document. The Nicaraguan election Reagan called for is scheduled for Nov. 4, but now it is unacceptable. Dissident Arturo Cruz, the likeliest challenger, says he cannot organize so quickly.

Junta leader Daniel Ortega, on the "Today" Show, inquired acidly if Reagan would give Mondale more time to get his act together.

The Sandinistas made their maddening announcement as l0 European foreign ministers were meeting in Costa Rica. Western Europe, since discovering that Reagan's prescription for peace required the mining of Nicaraguan harbors, has increasingly questioned his view of the trouble as "a fire in our front yard."

Mondale did not jump on this issue in his New Jersey attack on Reagan the mismanager. Mondale does not pounce on issues. He waits for others to speak first. On Lebanon, he followed his running mate, Geraldine A. Ferraro, into the fray. And on arms-control questions, he quoted copiously from columnists, even though Reagan's grave mistatements have long been on record.

His attack on Reagan as someone "who does not know what he believes" and a dreamer with wrong data was fervently received in the jammed State theater in New Brunswick. One Rutgers student was not clapping, however. Theodore Spanos, 22, a junior who came to the rally "to see if Mondale does anything but put down the president," said, "Blaming Reagan for that terrorist bombing is the lowest blow I ever saw."

There's one in every crowd. Reagan has reason to believe that there are millions in the country who just don't want to hear anything bad about him.