As Vice President Bush's motorcade wound through this sleepy Mississippi River town yesterday, it passed one black neighborhood after another -- past mothers sitting on porches with children in their laps, past old men gathered on street corners.

When he reached his destination, a gazebo overlooking the Mississippi River on the historic Natchez bluff, Bush was greeted by a virtually all-white crowd of several thousand men, women and children waving banners declaring "Four More Years," "America Is Proud Again" and "Mississippi Is All Yours."

The audience here, in a city half black by population, was typical of those Bush has drawn in the campaign, particularly on his last swing through the South this week.

His aides deny that the GOP has written off black voters, but they acknowledge that Bush is making little effort to reach them this week, despite the possible potency of the black vote in this region.

"The name of the game in the last week is shoring up your support," said Bush's domestic affairs adviser, Steve Rhodes. "If 90 percent of the black people say they're not with us, why do it?" Rhodes was referring to polls in Alabama showing that 90 percent of blacks support the Democratic ticket.

Monday, campaigning for a GOP congressional hopeful in Birmingham, where blacks also comprise half of the population, Bush went to a wealthy, predominantly white suburb and addressed an all-white audience at a local high school.

Later, a British Broadcasting Corp. reporter asked if Bush's choice of such a forum was "saying something to blacks" in Birmingham, a city whose history is laced with racial disharmony.

"We are saying something about where our strength is in this city," Bush said. "I hope there's not some subliminal message that says something to someone else."

Bush has made no appeals to white southerners, but the GOP strategy appears to play into old racial tensions in the Deep South. The Reagan administration has retreated from affirmative-action and civil-rights legislation embraced by the Carter administration, and the Democratic Party has tightened its links to the black community in this campaign.

According to a prominent Alabama Democrat who asked not to be identified, the Democratic Party, historically powerful in the South as the party of the whites, is now seen by many white and black southerners as the party of Jesse L. Jackson.

Mayor Emory Folmar of Montgomery, Ala., head of his state's Reagan-Bush finance committee, said the GOP saw a surge of white support in Alabama after the Democratic convention, and he attributed it to Jackson's prominence in the proceedings. Folmar said white southerners are uncomfortable not just with "the race thing" but also with Jackson's support of increased federal spending.

When Bush campaigned for House hopeful Sonny Callahan Monday before an all-white audience in Mobile, he drew boisterous cheers when he declared, "The party of Mondale and Ferraro and Tip O'Neill is not the Democratic Party that the people of Alabama remember . . . . The Democratic Party has left the people of Alabama."

Bush's aides said he was not referring to race and meant only to portray the national Democrats as big-spending liberals soft on defense and short on patriotism.

Bush has said repeatedly that blacks should support the Republicans because of Reagan's support for enterprise zones, black colleges and other programs that help minorities. But to many voters, the race issue appears inseparable from the overall themes of the campaign.

"We're for Ronald Reagan because we believe in putting people to work," said Joanne Wilson, a Republican who attended Bush's rally here yesterday. "There are an awful lot of black people in Natchez, but there are also a lot of people who don't want to live off the dole, don't want to take handouts and who want to have jobs."