The pros and cons of a Jesse Jackson candidacy are being hotly debated now that Jackson is the front-runner in Democratic polls.

According to one argument, Jackson shouldn't run because it is not possible for him to obtain the Democratic nomination. To that argument, Howard University political scientist Ronald Walters, a Jackson adviser, has a resounding answer: "Not true!"

Noting that Jackson won 3.2 million votes in the 1984 Democratic primaries, Walters estimates that if Jackson could pull 5 million or 6 million black voters and triple the 600,000 white votes he received in 1984 -- for a combination of from 7 million to 10 million votes -- he could possibly win the nomination in a multicandidate field. Walter Mondale won the nomination with 6.7 million votes in 1984.

A cynical answer to Walters, who wrote several position papers on Jackson in 1984, is that even if Jackson won the nomination, he would never be elected president. Furthermore, despite Jackson's deliberate efforts to broaden his appeal to whites, significant white support for Jackson still is lacking. While in the most recent Washington Post-ABC Poll, Jackson was supported by seven out of 10 black Democrats, only one of 10 white Democrats supported him. Yet Robert G. Beckel, a political consultant who managed Mondale's 1984 campaign, told The Post he is impressed by Jackson's white support, which is far more than Jackson drew in similar polls by the Mondale campaign in 1984.

That stronger white support partly reflects a careful Jackson strategy. Months ago, Jackson set out to court that vote and be more than just the "black candidate." Focusing on displaced farmers and industrial workers, young whites, homosexuals and left-out constituencies, he began addressing such mainstream issues as jobs, plant closings, education, housing, drugs, education and energy.

Although it is uncertain whether that appeal will work, seasoned political observers have noticed that Jackson is saying different things from what he said in 1984, and people are listening carefully. For example, he's calling on young white college students to take up the 1980s challenge to work for "economic justice" and telling Texans he's for a national energy policy that includes an oil import fee and exhorting all to "invest in America."

Furthermore, in deliberately tackling such mainstream issues as economic justice, he raises an umbrella under which a broad spectrum of U.S. residents -- including blacks -- can comfortably stand. Too often black problems are seen as separate from those of other U.S. residents. But Jackson shows that low-income whites, blacks and many other minorities have many problems in common. The difference is that black problems are often worse, attributable in part to income and race.

Yet, to the extent that Jackson's attempt to build a "rainbow coalition" does not work because of lack of white support, one question that must be asked is this: What would that failure say about white Americans who will reject a person, who is speaking most clearly in their self-interest, simply because he's black?

"It would say there are still so many racial blinders on people that they would vote against the only candidate who is treating them as a serious constituency," says Linda Williams of the Joint Center for Political Studies. ". . . And he's the most charismatic front-runner in either party."

Meanwhile, Jackson supporters are trying to correct some of the mistakes of the past. A major criticism was that Jackson's 1984 race produced very little for black Americans. His strategists are now saying they understand that the problem was lack of cohesion among black delegates, lack of organization, and inexperience.

"They handed me a script," said Walters, who ran Jackson's floor strategy at the 1984 convention. He learned too late that a convention is no place to do serious political bargaining, he said. "The Democrats wanted to put their best face forward. It was a made-for-TV convention."

To counter such problems, some strategists are proposing that black strategists pledged to all candidates convene in early 1988 to develop an agenda and common goals. Such a conference could help give a sorely needed organized discipline and focus to black voting power.

Whatever happens with Jackson -- and only time will tell -- he deserves a level course on which to run and to be treated as a legitimate part of the horse race. By making the preemptive judgment that he can't win and by failing to treat him as a serious and credible candidate, we count him out at the starting line. And that's not just a problem for Jackson, or for blacks; that's a problem for the United States.