John B. Anderson, the white-maned maverick who ran for president without a party in 1980, found his strongest constituency among the moderate, affluent, independent voters of the postwar ''baby boom'' generation.
This year, more than a third of the nation's voters are still independent, Mr. Anderson said at a recent breakfast with reporters. ''These are people that can go either way in '84.''
These are also the swing voters that Mondale needs to pull from the Reagan camp to win the Far West states of California, Oregon, and Washington. So Anderson - a former Republican congressman from Illinois who will no longer admit to membership in either party - is just finishing a campaign swing up the Pacific coast for the Mondale-Ferraro ticket, telling voters that President Reagan lacks competence in the treacherous arena of arms control and foreign policy.
Until August, Anderson planned to be campaigning for his own ticket this fall , under the banner of his own National Unity Party. He endorsed the Democrats instead.
But he may still launch his third party in 1988. ''It hinges to a great extent on what happens in November.'' A Republican landslide might weaken the Democratic Party, he says, to the point where a third-party effort might best be able to stem the rightward drift of the GOP.
The Mondale campaign here especially hopes Anderson can help it with young voters, who are favoring Reagan more decisively than any other age group. Anderson himself views Reagan's popularity without alarm.
''Youth?'' he says. ''Their opinions are very transitory. ''This is utter rubbish that the Republican Party is going to be the party of the future on the backs of today's youth. They could shift tomorrow.''
Reagan's popularity on college campuses grows out of a concern for job security, Anderson says. He argues that ''fortuitous events and not some cleverly designed macroeconomic policy'' have brightened the economic picture.
Anderson says he sees Democratic vice-presidential candidate Geraldine Ferraro as a powerful factor with moderate women - ''like an underground river'' that has not yet risen to the surface. ''There is a silent movement in American politics that has been dammed up for a long time,'' he says. It is not easily measured, he adds, in the polls.
In California, November's big electoral prize, Mondale is about 10 points behind Reagan in the polls. Democratic campaigners hope to make up the biggest chunk of that popularity gap by winning back disaffected, moderate Democrats in suburban Orange County, San Diego, and Santa Clara (Silicon Valley).
This is Anderson's basic constituency. He won 7 percent of the vote nationally in 1980, but reached a peak of 22 percent in popularity polls in July of that year.