And the terrain is tricky: Many of the states where the black vote could be most crucial are also those where Mr. Obama is deeply unpopular among many white voters. So Democratic senators in places like Arkansas, Louisiana and North Carolina must distance themselves from the nation’s first African-American president while trying to motivate the black voters who are his most loyal constituents…

The black vote could prove particularly decisive in four Southern states: Georgia and Louisiana, where African-Americans make up more than 30 percent of eligible voters; North Carolina, where they are 22 percent; and Arkansas, 15 percent.

While minority turnout traditionally declines in nonpresidential election years, there have been midterm elections in which Southern blacks played a pivotal role. An example occurred in 1998, when President Bill Clinton was, like Mr. Obama, under fire from Republicans and nearing the end of his White House years…

The Rev. Al Sharpton said listeners to his nationally syndicated radio show were increasingly focused on the elections. “People feel like they would be betraying the spirit of what happened in Ferguson, as well as enabling this impeachment rhetoric, if there’s a low turnout,” he said.