A full-fledged Democratic presidential campaign by a black candidate such as Jesse Jackson could alter the party's presidential nominating picture by significantly increasing the numbers of registered black Democrats and siphoning off support from front-runner Walter F. Mondale.

When Jackson was included in a list of Democratic candidates in the latest Washington Post-ABC News poll, he finished third, with 8 percent, far behind Mondale's 42 percent and Sen. John Glenn's (D-Ohio) 26 percent in a nationwide survey last week of 634 Democrats. But with Jackson out of the listing, Mondale jumped 5 points to 47 percent, while Glenn gained just 1 percent.

A small group of black leaders who urged Monday that a black candidate run for the Democratic presidential nomination also advocated a massive, concurrent black voter registration drive.

Questioned about his presidential ambitions in an interview with Post editors and reporters yesterday, Jackson said he thought he could put together a coalition that could make him the next Democratic presidential nominee.

He said there is growing acceptance of the idea that blacks can assert themselves politically through such means as running a presidential candidate and registering several million eligible black voters, particularly in the South.

"A few months ago people would laugh when you brought up the subject of a black presidential candidate," he said. "A few months ago you would have thought I was crazy if I said that Reynolds would go to Mississippi with me to look into registration violations." He referred to William Bradford Reynolds, assistant attorney general for civil rights.

But most of all, when talking about running for president, Jackson's emphasis was not on winning but on forcing white Democrats to "renegotiate a new relationship" with blacks.

Jackson said he has not decided to run, but that if he does, "I think it would expand the Democratic base, not divide it. Then it would be left to the white Democrats to decide if they wanted to renegotiate a new relationship with blacks that would be respectful and proper."

Jackson emphasized the need for white Democrats to begin putting blacks on party slates in proportion to their numbers for higher offices and seeing that blacks can be registered to vote in order to elect more black candidates.

Jackson said that white Democrats are still hampering black registration efforts in the South.

"The enforcement of the Voting Rights Act is the missing link to . . . our effort," he said. "In Mississippi, they are still operating under the Mississippi law of 1890."

He said that black voting power in the Mississippi delta region is being diluted by by gerrymandering, in which political jurisdiction boundaries are drawn to cut up concentrations of black voters and new federally subsidized black housing subdevelopments are being built just outside city election district limits.

Black voter power is also being diluted, he said, by a "dual" registration processes, in which registration is often available to blacks only in areas difficult for them to get to, where registration and elections are conducted in violation of the federal law and where voter registration efforts are allowed to continue under the old system even when communities have filed new plans that are awaiting Justice Department approval.

Jackson said that he had seen President Reagan on television in Mississippi on Monday, "and . . . there was jumping up and down for him. But no matter how high that crowd jumps, Reagan only won Mississippi by 11,000 votes" and there are about 200,000 unregistered eligible black voters.

Jackson also said he has had several meetings with Mondale advisers. Mondale's acting campaign chairman, James A. Johnson, said that the sessions have dwelled mainly on Jackson's voter registration crusade in the South, although there was some discussion of his possible candidacy for president.

"As near as we can tell, we do not think that Jesse Jackson has made any decision about whether to run," Johnson said. "But as far as the activity is aimed at increased registration of black voters, it will be of enormous benefit to our party."