Enter Jesse Jackson

Once you've said the Rev. Jesse Jackson's entry tomorrow into the Democratic presidential race expands the field to eight, it's hard to know what to say next. The Jackson candidacy admits many unknowns into the American political equation.

In the annals of black political progress, his presidential bid marks the second such serious effort for blacks, the first in more than a decade. Such milestones are worth noting. Yet even within the black community, whether the Jackson candidacy is the best strategy for achieving minority political aims is sharply debated.

Black voters themselves will have to decide whether a black candidacy in the Democratic Party best serves their interests - or whether they should invest their votes among white candidates who have a better chance of election. This is always the critical decision of voting blocs - whether the bigger potential prize from going it alone is worth abandoning the security of coalition.

And by taking the very step of announcing for a White House run, as a candidate in a party that is predominantly non-minority, Mr. Jackson is inviting whites to judge his candidacy on its wider merits - on his personal qualifications for the office, his character, experience, and judgment, without regard to color.

Mr. Jackson's lack of a specific political record makes it hard to judge his impact on the Democratic race. He has not held elective office. His background was in civil rights and in self-help education programs. He has been more a man of the pulpit as a preacher, more a social worker in civic life, than a politician. His credentials are chiefly those of an articulate speaker at public rallies, largely in minority communities, not those of a proven professional in legislative or executive chambers.

Few figures step confidently into the presidential arena without having held a major public office. Shirley Chisholm, the first prominent black to have run for the presidency, had served in Congress as a representative from Brooklyn before entering the party nomination fight in 1972 - a year, with Alabama's George Wallace running too, when race was more of a divisive issue than it is now.

Jackson's claim to credibility is the likelihood that he can benefit from, and in turn give shape to, the renascent black political movement in the United States. That movement was already under way independently of Mr. Jackson. Rep. Harold Washington's recent success in the Chicago mayoralty was one manifestation of the faster return to the voting booth among blacks than among whites since 1980. Still, the symbolic promise of a Jackson candidacy - that it might motivate substantially more blacks to register, and hence sweep into office blacks running at lower levels next November - has strong appeal. So does the contention that blacks would have more leverage on the Democratic Party through the primaries and at the convention next summer on issues of jobs and benefits and other matters vital to blacks.

The risks from an embarrassing voter reception for the Rev. Mr. Jackson should be taken into account, too. Mr. Jackson has been pulling less than half of potential black votes. In mayoralties, blacks successfully running for the first time typically win 9 out of 10 black votes, and 1 in 5 white votes. There is no way to know what to expect of black candidacies on the national level, however.

The conventional wisdom is that Mr. Jackson hurts Walter Mondale more than John Glenn. If Mr. Mondale wins the first tests in Iowa and New Hampshire, the horse-race analysts might discount the Mondale edge somewhat more by saying: Wait until next week in the South, where Senator Glenn is already stronger and the Rev. Mr. Jackson will cut into Mondale's base.

But the fact is, the Jackson candidacy will constitute a major experiment in American voting life. How much a factor race will be among blacks' and whites' decisions we will have to wait to see.