“She lights up the board demographically,” Mr. Paleologos said.
Boston is growing, according to recent census data, while its percentage of non-Hispanic white residents is declining, dipping from 47 percent in 2010 to less than 45 percent now. The city’s Black population is also declining, from about 22 percent in 2010 to 19 percent now. There is swift growth in its Asian and Hispanic communities.
Although Ms. Wu has benefited from a young, energized base — elements of the “Markeyverse,” which fueled the surprise re-election of Senator Ed Markey, reunited into a “Wuniverse” — she could encounter headwinds in the general election because of her positions on housing and development, like her support of rent control.
It is a departure, in itself, that so much of this race has centered on policy. Boston’s campaigns have long turned on ethic rivalries, first between Anglo-Protestants and Irish Catholics, then drawing in racial minorities as those populations increased.
Boston’s mayors relied so heavily on turnout from ethnic enclaves that they had no need to build a multiethnic coalition by presenting a bold vision, the way Fiorello LaGuardia did in New York, said the historian Jason Sokol, author of “All Eyes Are Upon Us: Race and Politics From Boston to Brooklyn.”
“They did not have to express any vision, nor did they end up governing with much vision,” he said.
The results of Tuesday’s preliminary election could guide the city into very different matchups for a November general election, including one that pits Ms. Wu against Ms. Essaibi George, who draws her core support from white neighborhoods.
Ms. Clark said she feared the battle between the two Black candidates could lead in that direction, closing a rare window of opportunity for the city, whose Black population is gradually waning with the rising cost of housing.
“I firmly believe, if Kim does not stay in there, we will not see a Black elected mayor in the city of Boston,” she said.