The conservative bandwagon has recently attracted some peculiar passengers, such as Bruce F. Caputo. He seems determined to offer vehemence in espousing conservatism as compensation for the tardiness of his conversion to conservatism. But his problem is veracity: he can't seem to get the hang of it.

Caputo, 38, wants the Republican nomination to run against Sen. Pat Moynihan (D- N.Y.). He is waist deep in derision because of lies he has told about himself. But equally bad is his mendacity about Moynihan, a kind of mendacity that is becoming common in campaigns.

Three months ago I asked Caputo if he really had once been drafted into the Army. I treasured his reply: "Not exactly."

After election in 1976 to his single term in Congress, Caputo's biographical entry in Congressional Quarterly said: "Military record: None." But soon his office biography read: "Served two years as an assistant to former Secretary of Defense Robert McNamara with responsibilities for budget analysis, legislative liaison and policy planning for SALT talks and Vietnam troop withdrawal."

Six months ago his Senate campaign committee said that at Defense he had been "responsible for . . . weapons procurement, legislation and the defense budget." As the public mood changed, he changed what he claimed to have been responsible for. In 1977 it was arms control and Vietnam withdrawals; in 1981 it was procurement.

He has repeatedly referred to himself as a "draftee" who served "a 21/2-year hitch" as a second lieutenant immediately after leaving school. He has made both claims while accusing Moynihan of being soft on defense. Both claims amaze the army's computer, which has never heard of Caputo.

Caputo now says he was not drafted "in the military sense of the word," and was not "technically a lieutenant." He says "I am not embarrassed" about having "misspoken" because he was a draftee in the sense that he sought civilian employment in the Pentagon to avoid the draft. He says he was "treated like a lieutenant" because he had to take orders, wear his hair short and get up early.

Today Caputo claims to be a deep-dyed conservative, but his coloration is the result of a recent rinse. In 1977 he criticized Jimmy Carter-- yes, Carter--for an arms-control proposal too "one-sided" and unfair to the Soviet Union. He worried that Carter might "rush to build new weapons systems" and "offend the Soviets." Caputo was one of only three Republicans to vote against the House defense appropriations bill. He also supported the sort of liberal domestic programs he now promises to "dismantle.">

Caputo has lost bids to become lieutenant governor and a Senate nominee, and is, surely, finished in politics. But before he sinks from sight, consider another, more common, campaign malpractice.

He distributed a letter calculated to curl the hair of conservative contributors by charging Moynihan with various sins. He said, for example, that Moynihan "voted for a plush $60 million Senate office building at your expense." Well, now.

The building was authorized in 1972, five years before Moynihan reached the Senate and began leading opposition to it. In 1979 he voted three times against additional funds for it, and even voted to rescind unspent funds already authorized. When that failed, he voted for funds attached to a cost ceiling. That is the small shadow of truth in Caputo's charge.

Caputo asserts that Moynihan "voted against capital punishment." Moynihan favors capital punishment for certain crimes. There has been no direct vote on capital punishment since Moynihan entered the Senate. Caputo is referring to procedural votes that reveal more about the parliamentary context at the time than Moynihan's views on the subject.

To untangle the truth from similar Caputo charges (such as that Moynihan favors foreign aid for Cuba, and voted to increase gasoline taxes and his own pay) would require many columns. That is the problem: as has been said, a lie can travel around the world while the truth is still lacing its shoes.

Not long ago, senators cast only 60 or so roll-call votes a year. Now they can cast that many in a week. Many of the votes are purely procedural, but can be misrepresented as expressing policy convictions. That tactic is increasingly used because there are so many votes, and so few opportunities for clearing up confusion once it is spread. The tactic can only be ended by an attentive press, and by an electorate that is as incensed when candidates misrepresent opponents as it is when candidates misrepresent themselves.