The push is an attempt to counter Republicans’ many advantages in this year’s races, including polls that show Republican voters are much more engaged in the elections at this point — an important predictor of turnout.

Image Representative John Lewis of Georgia has described Ferguson, Mo., as a rallying point. Credit... Doug Mills/The New York Times

Mr. Lewis is headlining efforts to mobilize black voters in several states with competitive Senate races, including Arkansas, Louisiana and North Carolina. The drive is being organized by the Congressional Black Caucus, in coordination with the Democratic Senatorial Campaign Committee. Other steps, such as recruiting N.B.A. players to help register more African-Americans, are also underway.

While Democrats always seek to increase African-American turnout, that they are taking such aggressive steps to rally their most loyal constituency reflects the increasingly difficult landscape they face. In recent weeks, seats in Colorado, Iowa and New Hampshire, once expected to tilt toward the Democrats, have become more competitive. Mr. Obama’s approval rating has tumbled below 40 percent in states with some of the most competitive races, and Republicans already seem assured to win at least three of the six seats they need to take back the Senate.

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.

Labor Day is the unofficial kickoff for the general election campaign, and analysts say the contest for control of the Senate remains unsettled, though most give a slight edge to the Republicans. After a turbulent summer dominated more by a succession of grim news events at home and abroad than by typical election-year appeals, officials in both parties agree that the campaign is fluid and that a wave election, with one party winning a large number of seats, is unlikely to happen.