Jesse L. Jackson refers to it as "challenging the people to have courage." His campaign manager, Gerald Austin, says the time has come to "call the question."

By whatever description, Jackson has begun to address two implicit questions about his candidacy: Is America ready to elect a black president? Is it ready to elect this black?

The wording of Jackson's responses varies, but the thrust is the same: If his supporters do not vote for him because he "can't win" -- code phrase for "is black" -- they are failing a moral challenge and wasting their votes. If they vote for him, they are affirming what is best about democracy by tearing down barriers for blacks and any group that has felt the sting of discrimination.

"If this was feudalism and my father was not king, then I could not win," Jackson said yesterday when the "can't win" question was put to him after a luncheon speech to the U.S. Conference of Mayors. "But this is a democracy, in which everyone has royal blood." As usual, he was applauded boisterously by a predominantly white audience.

Jackson says he has been forced to raise the issue because the media have perpetuated the "can't win" myth, refusing to take his prospects seriously even during the eight months last year when he led all the national polls for the Democratic nomination. (With Gary Hart's return, Jackson has been bumped to second place, although his share of the vote has continued to hover between 18 and 25 percent.)

His complaint about a media "hang-up" about his electability flies in the face of at least one content analysis of early network coverage of the presidential campaign. Robert Lichter of the Center for Media and Public Affairs, a nonpartisan academic watchdog group, has found that from February through October last year, Jackson led all candidates of both parties in the total of positive-minus-negative coverage on network news shows of his candidancy's substance. He trailed only Vice President Bush when the same measure was applied to "horse race" coverage.

Still, the "can't win" factor is on the minds of voters, journalists and politicians. Opponents and supporters raise it.

Last spring, for example, California House Speaker Willie Brown, who was neutral at the time, said that if he eventually endorsed Jackson, it would be based on calculations other than the expectation of victory.

"I think it is going to be a brokered convention . . . and I want to be in the room," Brown said in an April interview with reporters and editors at The Washington Post. "Jesse is the best way I know of to get a ticket." Brown said he believed Jackson had a lock on 1,000 delegates at the 4,160-delegate convention this summer in Atlanta.

Brown, who became Jackson's national campaign chairman last month, now he says he expects Jackson to win, and will not entertain questions about a brokering role. Brown is one of scores of black mayors, members of Congress and state elected officials nationwide who opposed Jackson in 1984 but support him now. One of the few holdouts is Detroit Mayor Coleman Young, who said at a news conference in November that while he agreed with Jackson's platform, "the trouble with Jesse is that he ain't never run nothing but his mouth."

Young has since softened his criticism. He said yesterday at the mayors conference that if Jackson can demonstrate in the early primaries that "the rainbow is something more than various shades of black," he would consider endorsing him. But he said that Jackson "can't win," and when asked why, responded ruefully: "Has racism improved between 1984 and 1988?"

There is some evidence that it may have -- at least in reference to Jackson. The securing of his base in the black community has freed him to spend most of the last year appearing before white audiences, with whom he has developed an extraordinary rapport. Of the Democratic candidates, he consistently draws the biggest crowds and best response.

Is this reaction to his celebrity and showmanship? Is it partly an expression of the guilt that some whites may feel about some whites' reluctance to vote for a black -- a guilt Jackson now may be playing on?

Rep. William H. Gray III (D-Penn.) said he believes it is something deeper. "Jesse isn't just playing on white guilt," he said. "There is a second-time-around maturity to his campaign. He is talking about broad issues and doing it very effectively."

Yesterday, before the mayors' group, Jackson added a wrinkle to his stump speech. He displayed a list of 50 corporations that, as a result of the 1981 tax cuts, made $57 billion in profits between 1981 and 1984 but paid no taxes and received $2.4 billion in tax rebates. The lost revenue, Jackson said, could have funded needed urban programs. "This," he said, fingering his new prop, "is the anatomy of a slum."

The audience, predominantly white, roared its approval. Was it because they agree? Was it because they think he can win the nomination? Or was it because they think he can't, and is therefore "safe"? When the voting begins next month, the questions will begin to produce answers.