As an emotional wave carries him toward a presidential candidacy, the Rev. Jesse L. Jackson is reaching a moment of truth about his future.

Jackson and some of his closest supporters are weighing more carefully than ever the pros and cons of entering the contest for the Democratic nomination for president. And the nearer they come to a decision, the more questions arise to make the choice difficult.

A group of black community leaders headed by Mayor Richard G. Hatcher of Gary, Ind., held the first of several regional meetings in Los Angeles yesterday to examine the possibility of starting a Jackson candidacy. The Rev. H. Hartford Brookins of Los Angeles said that the group hopes to "pin down" by the end of September a decision on whether Jackson should run.

Some are questions about a Jackson candidacy involve the nuts-and-bolts of mounting a national campaign--fund-raising, organization, complicated delegate-selection rules--although Jackson's advisers talk with increasing confidence about meeting those challenges.

But others are basic questions about what Jackson has accomplished as a free-floating advocate of a black candidacy, about the freedom he has had as a highly visible outsider criticizing the political establishment, about how he would be scrutinized if he crosses the line from gadfly to candidate and about whether his candidacy would cause an ugly internal battle among black leaders.

"I've been able as a non-candidate to operate in the gap and inspire people ," Jackson said at the annual meeting of Operation PUSH in Atlanta recently. "The campaign puts a lot of baggage on me. I have to weigh the freedom I would lose."

But Jackson and others also have begun to realize that it will be hard to turn off the emotion and anticipation building up among many blacks.

As the strains of "Where Would I Be Without the Lord" echoed at the PUSH convention, it was plain that if Jackson stops short of running, he will leave many loyalists disappointed and disillusioned.

Some who urge him to run say a decision not to do so could retard the drive to register black voters. "I think we lose if he doesn't run," said Mayor Johnny Ford of Tuskeegee, Ala.

But others say that the prospect of a black candidacy, coupled with a massive black voter-registration drive, has pushed the Democratic presidential candidates farther than ever in promising to consider the concerns of black citizens.

As a result, they appear genuinely torn--realizing that Jackson performs a vital role as a black spokesman but fearing that, if he does not enter the race, they may squander an important opportunity to mount a serious black presidential campaign. They also know that Jackson has the potential to excite what has been to date a lackluster Democratic contest.

"We don't want to lose him from what he is," said Brookins, a former PUSH board chairman. "We've been over that two or three times. But each time we say this is our chance to go with the larger picture, and if we go with it, we need someone to carry the banner."

Some of Jackson's supporters worry openly that many black leaders are still critical of a black candidacy, though they say there is now less open hostility toward the proposal.

Those critics include NAACP Executive Director Benjamin L. Hooks, Detroit Mayor Coleman Young and Atlanta Mayor Andrew Young. However, Andrew Young tempered his criticism while his city was playing host to PUSH.

Los Angeles Mayor Tom Bradley did not attend yesterday's meeting in his town.

Hatcher said he would like black leaders to meet to discuss Jackson's candidacy. "I think the people are where Jesse Jackson is. But I'm not prepared to see important black leadership overrun."

Hatcher's committee already has a fund-raising plan that could, the group believes, raise $10 million nationwide.

Asked why he is so confident when financial concerns have scared off other potential Democratic candidates, Brookins said, "They were running a campaign. With us, we're running a crusade."

After yesterday's meeting, Brookins said, "Today for the first time we got technically serious." He said the group put together a California Jackson Advisory Committee, headed by himself and with state Rep. Maxine Waters of Los Angeles as state chairman.

Brookins said those at yesterday's meeting will work to "identify sources of fund-raising and support for the Rainbow Coalition," the term Jackson is using for building a campaign backed by members of minority groups and women.

Jackson maintains that he will not be propelled into the presidential race on emotions alone. "I have not told you I was going to run," he said in Atlanta. "I have not announced and stopped as if I were a quitter. I have said all along it was the right thing to do, but I have reserved the right to determine the best and the most credible thing to do."