On the short list of Republican senators who voted against the MX missile--there were exactly six--the name of Gordon J. Humphrey of New Hampshire leapt off the page.

At a moment when the traffic was going the other way--91 House Democrats went down the line for the Gipper--Humphrey deserted President Reagan and his own previous pro-MX position.

This was the political unknown who in 1978 campaigned on a pledge to "reestablish military superiority over the Russian bastards." This was the man who became one of Reagan's most reliable hawks on the defense budget. And who recently gloated over the death of the freeze movement in several New Hampshire town meetings.

But there he was, standing with the lonely band of mavericks and peaceniks: Hatfield, Andrews, Durenberger, Stafford and Weicker.

He showed up his skeptical New Hampshire colleague, Warren Rudman, and such notable GOP liberals as Mathias of Maryland, Packwood of Oregon and Chafee of Rhode Island. They had all bought the argument that the MX was a bargaining chip with Reagan, who might otherwise not go to Geneva.

Humphrey's vote astonished his colleagues--Armed Services Committee Chairman John G. Tower (R-Tex.) is said to have aimed a glare in his direction "that would have reduced the desk to ashes."

The White House was prepared. The president had summoned Humphrey for a flattering private session, but, unlike so many susceptible Democrats, Humphrey resisted the claim that the MX would give Reagan indispensable "leverage" with the Soviets in the stalled Strategic Arms Reduction Talks (START).

The senator gave no hint to the folks back home of his imminent defection. He waited until just before the May 25 roll call to give his reasons for his dramatic turnaround on the "Peacekeeper," the Pentagon's pet name for the MX, and the "peacemaker" that Reagan had miraculously become for the rest of Congress.

Humphrey called the basing mode a "vice" that adds to the missile's vulnerability, which no one, from the Scowcroft commission to the lowliest Democratic convert, denies.

"I voted on the merits," Humphrey explained later.

If the rest of Congress had passed on the merits, the MX would have sunk without a trace. In the marvelous alchemy of Washington, however, the whole question had been transformed into a vote of confidence in Reagan.

Why would Humphrey, of all people, fail that loyalty test?

The Manchester Union-Leader, New Hampshire's far-right daily newspaper, suggested that he was not so much influenced by the merits as by the example of Norman E. D'Amours, the five-term Democratic congressman who could be the most formidable candidate the Democrats could field against Humphrey next year.

D'Amours spoke pointedly on the floor about Democrats who switched out of fear of being tagged with "having lost the arms race."

Humphrey has already responded to the D'Amours threat by evincing a new interest in acid rain, which, an announced Democratic rival, John Durkin, wickedly said, "Humphrey thinks is a rock group."

Humphrey, who impresses voters as a quirky outsider in the "Mr. Smith Goes to Washington" mold, has nothing to fear in a Republican presidential year, according to Tom Rath, treasurer of the New Hampshire GOP. Not much is being made of Humphrey's surprise vote on the MX, since the missile passed the Senate by a 20-vote margin.

He denies that there is any political content to his vote. He says he does not even know of the existence of a poll taken by his pollster, Arthur Finkelstein, which shows that the MX is not a winner in a state that gave the nuclear freeze its big push in 1982 town meetings.

Cloudy though it was, Humphrey's vote was the lone hint that a vote for the MX could be dangerous to a politician's health. The conventional wisdom is that the freeze movement has spent itself and that the new "build-down," which Defense Secretary Caspar W. Weinberger regards as ratification of Reagan's nuclear modernization, will be the standard to which the country will repair.

If Humphrey used the MX to wash out a campaign issue, a Democratic presidential candidate, Sen. Gary Hart (D-Colo.), has seized it to revive his faltering hopes.

Hart has lost custody of the peace vote to a rival for the nomination, Sen. Alan Cranston (D-Calif.). The true believers find an equality of commitment in Cranston's single-issue candidacy. His bald head, his uncharismatic speaking style--"he sounds like a recorded announcement," they concede--concern them not at all.

Hart has challenged colleagues to join him in a filibuster against the second stage in the MX blastoff, the authorization bill. He thinks the recruitment of six or seven lukewarm MX supporters could make a difference.

It might be too much to hope that Gordon Humphrey could join George McGovern's former campaign manager in a floor fight to stop the MX. But nothing logical has occurred in the controversy yet.