Washington's never-ending quest to make the art of politics an exact science marches on with the advent of a new weapon now in the arsenals of liberal Democrats. In the trade it's called Claritas, and in the hands of political consultant Matt Reese - and a couple of his friends - Claritas is a startlingly precise technique for pinpointing supporters and opponents of issues or candidates.

Developed for commercial purposes in the early 1970s by Washington statistician, demographer and computer wizard Jonathan Robbin, Claritas is a system of catergorizing citizens by neighborhood groupings called clusters.

Using census bureau data and the theory that "birds of a feather flock together," Robbin defined 40 basic clusters he says embrace most types of American neighborhoods. For example, cluster 25 is defined as"urban/suburban, older, upper-middle with a substantial Jewish segment." Cluster 38 describes a community of "middle-aged Southern farmers with a Spanish migrant element."

Robbin then finds about 280 households he calls block groupgs and assigns them a cluster designation. A fundraiser, salesperson or politician then ccan trarget his appeal, sales pitch or political message to his audience.

"What this means, I think, is that some Democrats will get elected in 1980 that otherwise might not," says Reese, whose trademark in past campaigns has been a zealous pursuit of two kinds of peope: undecided voters who could be persuaded to support his candidate and favorable voters who need prodding to get to the polls on election day.

"If you want to pick cherries, go where the cherries is," he is fond of telling organizers.

Bill Hamilton, a favorite pollster among Democrats, calls Claritas "a highly effective tool for staying power or knocking off an old curmudgeon." Hamilton fits in the picture this way: he can poll a sample group of voters in a state and tell Reese what kinds of people - rich, young or religious, for instance - think of a specific issue or candidate. That allows Reese to tailor his campaign strategy to appeal to whomever he wants to reach.

"Polls by themselves are valuable but inadequate," says Reese. For example, a poll may tell a campaign consultant that working-class Catholic housewives aren't favorably impressed with a cadidate. But "the only time you know a woman is Catholic is on Ash Wednesday," says Reese. Now Claritas can specify block groups most likely to contain working-class Catholic women.

Although magazine, book club and direct sales firms have been using Robin's creation for selective targeting over the last several years, Reese was the first person to put Claritas to a political test. The place was Missouri.Reese was hired last year by a coalition of labor groups. His assignment: to defeat a question on the Missouri ballot that asked if the state constitution should be amended to outlaw union shops.

The opposition, the right-to-work advocates, seemed to occupy the catbird seat. Ree's own poll, taken by Hamilton, showed 69 percent of the voters favored the amendment, 30 percent were opposed. Even among labor union families the measure was favored 57 to 35 percent.

Reese decided to spend only 15 percent of labor's $2.5 million war chest on media - commercials can stir up the opposition, attracting opponents who might not otherwise vote. Instead he concentrated on registering citizens likely to vote his way.

Waves of postcards, telephone calls and personal visits bombarded households identified by Claritas as likely "cherries" ripe for Reese's picking. The quite, directed attack worked - 60 percent of those who voted rejected the amendment. Reese estimates that 110,000 new voters registered in Missouri that season registered because of labor's efforst.

Smart pols everywhere took notice.

"Never before," says pollster Hamilton, "has anyone had a situation where a pollster can tell a campaign, 'Look, we have problems with people who live in new towns, who are white and upper-middle class,' and then tell the guy that 40 of these people live in this county, 60 here and 120 in the northwest part of the state. And here are their names, addresses and phone numbers.'"

Robbin's firm, Claritas Corp. has agreed with Reese that Claritas' computers will only be used on behalf of the kind of clients Reese likes to help. Which means if you're a Republican who thinks Claritas sounds terrific, tough luck. CAPTION: Picture, no caption, By Joel Richardson