It was a lovely morning, and some people were rejoicing that common sense had come at last to Washington, at least to the House of Representatives, at least in the matter of the MX.

Then word came that a man with a "Ban Nuclear Weapons" sign on his van was threatening to blow up the Washington Monument with explosives that he said were inside the vehicle.

It probably made some sort of sense to this man that dynamiting the most conspicuous column in the heart of the capital would compel others to share his obsession.

But what the House debate the day before had illustrated was that many people in high places, who would never dream of dealing with explosives, are not exactly rational, either, about nuclear arms.

Many members were unable to see the logic of the administration proposition that the way to stop the arms race is to build more arms, specifically a $26 billion orphan missile with no place to lay its warheads. The MX has changed its name to "Peacekeeper," and a hundred of them have been booked into a mass grave in Wyoming.

Rep. Jim Leach (R-Iowa) said the strength of the "Dense Pack" basing plan -- which the Pentagon now has rechristened "Closely Spaced Basing" -- is that "it is so awesomely dumb."

Rep. John Conyers Jr. (D-Mich.) said that the unstated issue in the swirl of arguments was "whether we are sane or insane."

The House verdict was overwhelmingly -- 245 to 176 -- against the lethal Peacekeeper.

The White House received the news with grim warnings that members were "sleepwalking." The president will "take the issue to the country," it was said.

It already has been brought to the country.

The last time the House voted on MX, in July, it won by three votes. The intervening event was the Nov. 2 election, which, many survivors thought, had been a clear call to subject the Pentagon to the same excruciating examination of its use of government funds as, say, a rheumatoid arthritis victim who is required to justify Social Security disability payments.

Also, nine states had voted for a nuclear freeze.

The friends of MX argued in vain that the monster system is indispensable to our START negotiators in Geneva. But yesterday morning, as the Park Police were rounding up reporters to talk to the would-be bomber at the Monument, our ambassador to the Geneva talks, Edward Rowney, was telling the Senate Armed Services Committee that to him MX is "not a bargaining chip."

And Rep. Joseph P. Addabbo (D-N.Y.), the unlikely middle-aged peacenik from Queens who led the fight against MX, said the danger was not to treaties that might be, but to those that are -- ABM and SALT.

He pleaded against "giving our children the heritage of seeing this Congress start down the road towards the abrogation of the SALT agreements intended to stop the escalation of this nuclear insanity."

MX played to an almost empty House -- never more than 50 on the floor, and that is counting aides, members' children and two ex-congressmen.

Rep. J. Kenneth Robinson (R-Va.) complained that the poor attendance showed "how many people have already made up their minds."

They had, and for a variety of reasons, including perhaps the anticipated difficulty of explaining back home why they had spent $26 billion for a weapon of dubious value while constituents cower at rumored administration offensives against the unemployed, the sick, the old and the cold (emergency fuel assistance is under scrutiny).

Several Republicans deplored the "levity" of the discussion. They invoked Pearl Harbor.

Rep. Marty Russo (D-Ill.) brought down the House when he said that the "original Dense Pack occurred at Pearl Harbor when some military geniuses decided that the only way to make sure the planes would be safe was to cluster them on the runway."

At the morning-after Senate hearing, while hostages were being held inside the Monument, the president's science adviser, Dr. George A. Keyworth II, was describing the "fratricide phenomena," the comforting theory which says that incoming Soviet missiles will obligingly blow each other up. He told of the intense radiation, the nuclear blast wave, the fire ball, the "tremendous cloud of debris extending to great altitudes."

"Even small particles in the air will cause tremendous damage to an incoming warhead," he said. And what will they do to Wyoming? And what will they do to nearby states as well as to "nearby warheads?"

Such talk drives a lot of people crazy. Not all of them threaten to blow up the Washington Monument. Some vote against MX.