The return of the high-flying Reagan campaign to reality was imposed less by the rapid change in the polls following saturation coverage of the Democratic National Convention than by Ronald Regan's inexplicable turning back to the Vietnam War as "a noble cause."

What is stunning about that self-inflicted wound was the Republican nominee's insistence on rewriting those words back into his Veterans of Foreign Wars text. They had been excised by one key adviser, encouraged by others, who ran a red pencil neatly through an early speech draft hoping to stop this resurrection of one of the most emotional and controversial chapters in American history.

Who did clear the final draft of Regan's Aug. 18 Chicago speech, headlined on page one of the Atlanta Constitution: "Vietnam was 'noble cause,' Reagan says"? The answer: Reagan himself.

"I was appalled," one Regan operative told us. In Regan's political headquarters in Alexandria the day after the speech, another search was on to define and execute a "fail-safe" process that will lower future risk to an irreducible minimum. Reagan's irresistible urge, in the words of one intimate, "to tell it like he feels" was the major strategic problem on the desks of Bill Timmons, Reagan's national political director, and campaign consultant Stuart Spencer.

Clearly needed, as so often in the past, is a tough political strategist to say "no" to Reagan when his self-destructive urge gets out of hand. That could be Spencer -- or it could be Jim Baker, a rising influence in the Reagan camp.

Reagan's intellectual honesty in decrying what he called the "feelings of guilt" over Vietnam is not in question. It is widely shared, and not only by Reagan's partisans. In the southern states, where President Carter faces a stiff challenge from Reagan, there is particularly strong and patriotic support for his statement that the U.S. effort in Vietnam to turn back "aggressors bent on imperialistic conquests" was indeed a "noble cause."

But the great majority of those voters need no reminder of where he stands. They know. For many others, however, the reversion to Vietnam reopens old wounds beginning to heal. It was a sledgehammer reminder of old anxieties about Ronald Reagan.

The "noble cause" sentiment that came from Regan's soul illustrates a potentially dangerous insensitivity on his part: inability to understand the automatic and highly emotional reactions of these other voters. Without that sensitivity, his own political-alarm system failed to sound urgent warning bells.

Even if a "fail-safe" mechanism for Reagan is found by his advisers, it will be hard to plug in to Reagan's off-the-cuff comments and in the huly-burly of press conferences where he lacks a written text. Press secretary Lyn Nofziger had that in mind when he told Republican state chairmen in St. Charles, Ill., Aug. 12: "Don't ask for press conferences when we come to your state, or set any up."

Nofziger, a tough-minded former newsman himself, laid it on the line: press conferences are campaign weapons that should be sparingly scheduled not to please the press but to benefit the campaign; they are not for the press to ask questions "willy-nilly" but to move the campaign in the direction Reagan and his strategists want.

Controlling Reagan's weakness for self-inflicted wounds in the off-the-cuff replies to press conference questions, in other words, is to be attempted by curtailing his meetings with the press, an old and hoary tactic in the campaign bag of many previous presidential candidates. It may or may not work. Newsmen will battle for more access.

What can and must be made to work for Reagan, however, is a "fail-safe" mechanism to take the boomerang out of prepared speeches that are supposed to advance Reagan's cause at Carter's expense, not vice versa.