Standing beneath a banner that read, "Black America: A People Whose Time Has Come," Mayor Richard G. Hatcher of Gary, Ind., today helped fuel a possible presidential candidacy by the Rev. Jesse L. Jackson at the annual PUSH convention here by laying out a strategy for primary season: win the politics of expectations.

Hatcher, chairman of a committee that is exploring a black presidential candidacy, said that even a modest showing in the New Hampshire primary would generate, in his words, "Big Mo" (momentum) because "no one would expect a black candidate to do anything."

"What would folks expect a black to do in Iowa or New Hampshire?" he asked a cheering crowd of 1,000 delegates. "Nothing. But what if we did something--almost anything--where they expected us to do nothing? It would be viewed as the greatest thing that ever happened."

Hatcher advanced this idea shortly before three declared Democratic presidential candidates--former vice president Walter F. Mondale and Sens. Alan Cranston (D-Calif.) and Ernest F. Hollings (D-S.C.)--appeared at a question-and-answer session with a panel of reporters.

All three said they would consider a black running mate and supported the idea of a black presidential candidate. "Anyone who's an American ought to be free to run anytime they want," Mondale said.

"Run, Jesse, run," said Hollings, echoing earlier chants by the more than 2,000 people in the hotel ballroom. But Hollings added that he was "worried" about a campaign by Jackson because "he'll take black votes away from me in South Carolina."

Asked if he would consider a black running mate, Mondale said: "Yes, I want to find the very best vice presidential candidate in this country, and I've got to look at black Americans, brown Americans, male and female."

All three men attacked President Reagan's civil rights record, pledging beefed-up enforcement of the Voting Rights Act if elected.

After the forum, Jackson declared that the three guests had gone "qualitatively beyond where any Democratic nominee has ever been before" in advocating a possible black candidacy. But he said there was still a "gap" between all the announced Democratic candidates and him over the rights of the disenfranchised.

Some blacks fear that a black presidential candidate could tip the Democratic nomination to a moderate candidate such as Sen. John Glenn (D-Ohio) by siphoning off black support from a liberal such as Mondale.

In meetings with Jackson's advisers, Mondale strategists have outlined problems of finance and organization and the Democratic Party's "threshold" rule requiring a candidate to win at least 20 percent of the vote in a primary or caucus to win any delegates.

But Hatcher likened a black candidacy to Don Quixote, who "dared to dream the impossible dream."

"Some say it may be getting late. But it's not too late," he thundered. "Late August or early September will be early enough . . . to put together a serious national campaign."

Jackson, who has yet to make up his mind, said he will decide by then.

"Some say the process is too complicated," Hatcher said. "That's like telling us we are too ignorant and too stupid to handle anything more complicated than 'two plus two.' Others say Jesse Jackson is $1 million short."

But Hatcher projected 40,000 people giving an average of $250 to a "serious presidential campaign," which would raise $10 million plus federal matching funds.

"If George Wallace could raise $10 million in 1968, we can raise $20 million in 1984," he said.

In March, nine southern states that have a large black vote that might be turned out for Jackson have scheduled caucuses or primaries within an 11-day period, Hatcher said.

A massive voter drive aimed at signing up many of the 2 million unregistered blacks in the South would benefit a black candidate, Hatcher said.

Then, big midwestern cities like Chicago, where an enormous black turnout helped elect Harold Washington the city's first black mayor, would also support a black presidential candidate.

"We have been dreaming in the daytime, but scheming in the nighttime," Hatcher said.

The Rev. H.H. Brookins, an Episcopal bishop from Los Angeles and the chief organizer of black ministers for Jackson, announced a nationwide petition drive to draft the founder of PUSH. He said 125 ministers met three weeks ago in St. Louis to plan strategy.