A political maxim of Gov. Jerry Brown of California holds that "a little vagueness goes a long way in this business." Brown ought to sit at the knee of Rep. Barney Frank, the Massachusetts Democrat whose political style suggests that a little exactitude can go longer. Frank is a thumping liberal and at 42 has yet to have a feeling of abashment about his conviction.

Frank is one of 12 members of Congress who because of redistricting are in reelection fights against other incumbents. Either he or Rep. Margaret Heckler, an eight-term Republican and one-term Reagan loyalist, will be disseated. With the president bugling the message that liberal Democrats of Frank's ilk have created the mess that Reagan claims he can get us out of, few congressional races are better test cases of whether the public is buying that argument or gagging on it.

First, Frank's liberalism. If politics were theatrics, Frank would be superlatively cast as the authentic liberal. Not only is he comfortable with the script passed along by earlier liberals like Robert F. Kennedy, Philip Hart and Allard Lowenstein, but he also has avoided the new--and useless--sideshow of neoliberalism. The latter has validity only if the premise is accepted that the country has turned conservative.

In his office the other day, Frank questioned the Rightward-Ho argument. "The country is liberal in specifics. People say 'no' when you ask 'are you a liberal?' (But) people take the liberal side on environmental issues, the liberal side on Social Security, the liberal side on student loans, the liberal side on Medicare."

The Frank side has freshness to it because this son of a New Jersey truck-stop operator, this Harvard graduate with two degrees, is artful at making his points with wit. He acknowledges that Reagan's attacks on liberals have helped him. After Claude Pepper, the 82-year-old liberal running for reelection from Florida, campaigned for Frank in Massachusetts, Frank told a gathering in Washington that was celebrating Pepper's birthday: "I'm here to salute one of the two senior citizens who have made it possible for me to be a viable candidate for Congress this year -- Claude Pepper. The other is Ronald Reagan."

Frank has another one-liner: "I'm probably the only guy in America who makes less than $100,000 a year who's been a beneficiary of Reaganomics."

And one more: The Reagan administration "wants to give a means test for Medicare. These people like means tests. They're very good at them. No matter how you test them, they come out mean."

Frank, who has few moments of ponderosity in a profession enswamped with it, has the gagman's quickness for humor. His suits hang on his bulgy frame like overfilled Hefty bags, and he is to fashion what Tip O'Neill is to physical fitness. Frank's district includes the four miles of Heartbreak Hill in the Boston marathon. But he will keep his girth and cigar, thanks, and ride the press bus on race day.

Frank came to Congress after eight years of liberalism in the state legislature, and he plans to go all the way with it in the last weeks of his campaign. Heckler doesn't appear to know how to handle him. She tried the trashing approach. In a recent debate -- at the end of it when the rules prevented a Frank reply -- she said he favored pornography while in the legislature. Actually, Frank had sought to set boundaries for an adult entertainment zone. The Boston Globe pointed out that police and prosecutors supported the bill, because it was designed to contain, not spread, pornography. Aiming poorly, Heckler slung her mud, and when it missed it splattered on the wall and sullied her.

Unlike some Democrats, Frank is not dumping on Reagan as the sole cause of what ails America. He concedes that Reagan inherited a weak economy, but that the president and his supporters like Heckler are accountable because they have worsened it. Like the president, Frank would like the voters to "stay the course" -- with the liberal specifics that the public has been supporting all along.