Politics, which is definitely an art and not a science, is probably the most imitative of all public activities. Politicians return faithfully to those feints and formulas that have worked successfully for them (or other pols) in earlier battles.

A winning political formula has no expiration date; it is considered timeless. That may explain why, in Massachusetts at this very moment, there are easily two dozen Democratic candidates seeking office, all of whom are unnaturally pronouncing "again" so that it rhymes with "rain" because that's how Jack Kennedy said it. Changed circumstances did not inhibit Ronald Reagan, in the second year of his first term, from disparaging Washington as the home of the "puzzle palaces on the Potomac," a phrase he had used in 1966, when Lyndon Johnson, not Reagan, was the capital city's first citizen.

Some such duplication can be funny, and much of it is probably harmless. But occasionally, in that enclave of insecurity called national politics, there occurs a case of really bad copying. That's what the "Progressive Political Action Committee" is guilty of in its obvious aping of the National Conservative Political Action Committee: NCPAC.

Calling itself PROPAC, the new group seeks the support of people who are "sick and tired of negative politics," which by now must include a majority of those both above and below the age of reason.

But PROPAC turns out to have a peculiar definition of non-negative campaigning. Its initial advertising includes unflattering caricatures of conservative senators, featuring one of Sen. Jesse Helms (R-N.C.) as a flashlight-toting cop peering about a couple's bedroom. This ad, which is an attack on Helms' "human life" bill, may contain the most candidly selfish and offensively materialistic argument ever made for legalized abortion: "The Helms bill would require an additional $40 billion per year in welfare payments because of children born to welfare mothers who otherwise would have an abortion." That must represent a new strain of compassionate pragmatism. Apparently, when it grows up, PROPAC hopes to be an anti-conservative clone of NCPAC.

While that may or may not be an appealing prospect, it is not a modest ambition. NCPAC, which collected more than $7.6 million for the 1980 campaign, contributed less than $238,000, or 3 percent of the total, directly to candidates or their campaigns. Preferring to hurt candidates it opposed rather than to help candidates it favored, NCPAC spent some $1.2 million for the purpose of defeating six incumbent Democratic senators in the last election. Voters in those states were told that, because of their senators, "crime continues to rise . . . our nation's moral fiber is weakened by the growing homosexual movement, by the fanatical ERA pushers (many of whom publicly brag they are lesbians) by leftist-produced movies and television programs that are often indecent and full of sex."

NCPAC's recipe was simple: for vision, substitute venom; for reason, hate. But four of the six Democratrs lost, and in politics, success spawns imitators.

Those who objected in 1980 to NCPAC's savaging of senators George McGovern (D-S.D.) and Frank Church (D-Idaho) through the legal fiction/political travesty called "independent expenditures" better be ready to explain why such attacks are suddenly acceptable in 1982, when those being trashed belong to the other party.

Good campaigns are about differences--differences between candidates and their parties over their values. Good campaigns can and usually do include some negative charges. But the process and the public are both better off when those charges are made by the people whose names are on the ballot and not by any hit-and- run artists of any political persuasion, hiding behind the dodge of independent expenditures. One NCPAC is already one too many.