JOHN NANCE GARNER, who may have made less of an impact on American politics than any high ranking U.S. official in the twentieth century, is best remembered for his comment that the vice presidency--a post he held anonymously under FDR--"isn't worth a pitcher of warm spit." Walter Mondale, conscious of Garner's feelings, insisted in 1976 that Jimmy Carter let him be an "activist" vice president. "Activist" meant he didn't want to be treated like another former vice president and Mondale's political mentor, Hubert Humphrey, who found his way into the Oval Office as often as a lost tourist who had strayed from the morning tour of the White House.

In fact, Mondale has become a key adviser to Carter--a rather dubious honor in recent times. Widely regarded as unusually intelligent and a smooth operator on Capitol Hill, Mondale's main fault appears to be that his loyalty to Carter occasionally outdistances his better judgment. And in many respects, the cool, detached liberal from Minnesota is still somewhat of an enigma in the Carter administration.

Finlay Lewis, now the Washington bureau chief for the Minneapolis Tribune and a journalist who has covered Mondale for a decade, goes a long way toward unraveling the mystery of the Minnesota Fritz. In his unusually candid and balanced portrait, Mondale emerges as a man of unusually good political fortune who knows how to take advantage of the many opportunities that roll his way. Clearly he is a specialist in backroom politics, and that may account for the fact that he was appointed to nearly every significant post he has held. His liberal idealism is tempered by a well-developed sense of political self-preservation; he knows when to shut up. He would rather compromise and build a coalition of moderates than stand out on a limb with extremists. And above all, Fritz Mondale lacks the limitless politics ambition that governs the actions of so many of his colleagues.

IN THAT QUALITY, Mondale was sharply contrasted with his fellow Minnesotan and erst-while mentor, Hubert Humphrey. Mondale entered politics at the grassroots in the sometimes powerful, often self-destructive Democratic Farmer Labor Party (DFL) in Minnesota. It was in the DFL that Mondale first linked up with Humphrey. His awkward, sometimes competitive relationship with the fiery liberal was one of the most complicated aspects of his complicated personality.

Humphrey and Mondale appealed to the same constituencies, and one wonders if Fritz was appointed to fill a vacancy as Minnesota's Attorney General partly in an effort to get him out of the way. But in 1964, Mondale successfully emerged from state politics when he was appointed to fill the Senate seat that Humphrey vacated when he was sentenced to four-and-a-half years as Johnson's V.P.

Humphrey had a lot of time on his hands to help the freshman senator after the election in late 1964, and Mondale lacked identity independent of Humphrey for several years. As Humphrey's protege, he felt he could not criticize Johnson's war policies, and he later regretted it. For several years, he languished in the Senate as Humphrey's mouth-piece, and didn't find an issue of his own until 1968 when, against the wishes of the Democratic leadership, he put together a remarkable coalition to pass the Open Housing legislation that was essential to the civil rights movement. From that point forward he became a vocal champion of liberal causes, and with Nixon in the White House he no longer had to sit on his hands. He flirted with the idea of a run for the presidency in 1976, but eventually decided he didn't have the fire in his belly to fight for the nomination.

MONDALE GAINED from his experience the tools to be a genuinely active veep. Fritz Mondale will go along and get along unless he is pressed to the wall. Then he unleashes some awesome legislative power to get his way. The other lesson of his terms in the Senate is something his present boss obviously has not learned: You have to conserve your power to obtain votes for the really big issues. Lewis claims that Fritz tried to make that point to the Georgia Mafia early in the administration, but no one was listening. Instead, Carter flooded Congress with legislation on every conceivable topic, and much of it is still gathering dust in Congressional file cabinets.

Mondale's greatest weakness is that he doesn't speak up often enough. He has saved Carter from more than one foreign policy blunder, but now one senses that both are in over their depth. He values loyalty a little too highly, and perhaps he fears that if he makes too much noise Carter will send him to the little cell in the bottom of the Executive Office Building where all the past vice presidents have been locked up.

Lewis is quite direct in criticizing Mondale for his political misjudgments and praising him for his numerous victories. He has written an insightful analysis of the psyche of a man who is probably overqualified for his job. One can only hope that his otherwise successful career won't drown in the vice presidential spittoon.