Sen. Thomas J. McIntyre (D-N.H.) is not sure what happened to him Nov. 7, but one thing is certain: he plans to write a book about it.

He has already begun. The book concerns what McIntyre calls "the radical right," and the author promises that it will be a forceful indictment of people he feels are not conservatives at all, but radicals "who profess to love this country more and understand it better than you do or I do," in the words of his introduction.

McIntyre was defeated by the voters of New Hampshire Nov. 7. They decided to replace him with a little-known airline pilot with no real roots in New Hampshire, Gordon Humhphrey, 37. Humphrey was state chairman of the Conservative Caucus in New Hampshire, one of the organizations McIntyre classifies as an element of the radical right.

A deep vein of irony runs through Humphrey's victory. Last March, in one of the rare moments of rhetorical excitement in the long debate on the Panama Canal treaties, McIntyre fairly thumbed his nose at the far right.

"My political fate is not my concern," McIntyre told the Senate on that occasion, when he announced his decision to vote for the treaties. "My concern is the desperate need for people of conscience and good will to stand up and face down the bully boys of the radical new right . . . "

Today, McIntyre's friends and associates seem to believe almost universally that McIntyre's defeat in November was a consequence of his vote on the canal treaties. "Tom gave them the one thing they needed - a handle," one friend said recently.

Humphrey and his supporters seized the handle with zest. With the active support of William Loeb's Manchester Union-Leader they hammered away at McIntyre not only for his canal votes but for allegedly causing inflation, hiding the truth about the decline of American military strenth, and other reputed transgressions.

In a recent telephone conversation, Humphrey said that McIntyre's Panama vote and his "diatribe on the Senate floor" were crucial factors in his suprising 6,000-vote defeat.

Humphrey acknowledged that he is part of the "new right," but said McIntyre's efforts during the fall campaign to depict him as a far-out kook had failed "because I am not a kook . . . I'm a civil and gentlemanly person"-and, he added, a "strong conservative."

McIntyre seemed to agree, noting in an interview that Humphrey seemed to be a "nice enough guy, I guess." In fact, Humphrey's victory came as a total surprise to the senator, who had served 16 years in Washington and was prepared for six more. All the polls and professionals had agreed that he was a shoo-in; only the voters demurred.

But McIntyre's concern with the far right was not just a product of his defeat by Humphrey. He had begun to write his book-tentatively called "The Fear Brokers"-before the election, and, as he likes to point out, he has been fighting the right for years in the persons of publisher Loeb and Gov. Meldrim Thomson Jr.-who, to add to the irony, was defeated by the same electorate that picked Humphrey over McIntyre.

The early chapters of McIntyre's book, written before his defeat, contain repeated warnings that the right wing is becoming more powerful and more ingenious. The voters of New Hampshire made those warnings a lot more plausible than they might have sounded if McIntyre's right-wing opponent had not gone on to oust him from the Senate.

"Its the scare technique," McIntyre said the other day, sitting at his senator's desk, although all his senatorial photographs had been removed from the walls. "They oversimplify," he added, "making up slogans" that reduced complex issues to catch-words and phrases. "I've always been plagued by the grayness of issues," McIntyre said, "but these guys know they're right, inalienably right. I think it's dangerous."

He saw scare tactics in his opponent's campaign, McIntyre said, when Humphrey criticized the Social Security System and called for phasing it out in favor of private insurance plans.

"God knows we have some problems with Social Security, but it's such a part of the fabric of our nation, I think 140,000 people in New Hampshire depend in whole or in part on it. That's a scare technique, because if you're a little old lady sitting up on the sixth floor of a hotel in Denver and you're depending on that check and you hear somebody on the radio saying it's in bad shape and we ought to replace it, it's a worrisome thing. It's the scare technique, it's the fearmongering.

"I think that they're going to do better and better as long as we have inflation," McIntyre said of the new right. "Inflation is a menace to everybody, and as long as people are confused about the purpose of government, and they read what you fellows tell us about what a bunch of bastards we are down here, how we're cheating," they'll be vulnerable to new right rhetoric.

Most McIntyre talks about these things coolly, but his anger shows when he recounts the personal charges made against him (most often by Loeb's Union-Leader) over the years.

"I see them as vicious in their attacks," he said. "We felt it very much, a lot of us, on the Panama Canal treaty. 'You're a Benedict Arnold'-they'd make speeches and say, every senator who voted for those treaties is a Benedict Arnold, betrayed his country.

"I've been criticized for a lot of things, but that's hitting very low," things, but that's hitting very low," McIntyre said quietly but bitterly.

McIntyre says all the slogans and simplifications have a single purpose. "They do it to gain political power. They realize that the easy answer is the slogan, the quick way to the minds of our voters. These people are simply building hysteria into the issues in order to be able to gain their votes and to obtain their end, which is political power."

From Gordon Humphrey's point of view, McIntyre is making a monster out of a simple-enough phenomenon; "A younger generation of mostly Republicans, mostly from working-class backgrounds . . . a movement of people who are adopting the tactics of the old left, if you will," to win elective office. "It isn't at all sinister or evil or under cover," according to Humphrey.

Does McIntyre regret his Panama votes, or his challenge to the new right of last spring? No, he won't say that. It was the right vote, he is certain. And then a moment of candor:

"Now listen, I don't want to be a hero. I thought we were going to beat this guy by 30,000 votes. Any my polls told me we were going to. So when I made that speech I knew in my heart and soul that it was something that Mr. Loeb and his friends could really tear me up on. But he's tore me up before. I've been throwing the mud back for a long time..."

In other words, it wasn't supposed to turn out this way. Tom McIntyre figured to win. CAPTION: Picture, no caption, UPI