Hillary Clinton will announce on Tuesday that 50 current and former black mayors are backing her, including Michael Nutter of Philadelphia, Kasim Reed of Atlanta and Steve Benjamin, the mayor of Columbia, South Carolina and president of the African American Mayors Association.

The endorsements are the latest illustration of Clinton's strong support among African-Americans, who favor the former secretary of state over Vermont Sen. Bernie Sanders by over 50 points in many polls. And the endorsements are another sign that many elected officials in the Democratic Party are backing Clinton without waiting to see if Vice President Joe Biden opts to enter the race.

Clinton has made courting African-Americans activists a key part of her campaign, from attending the annual gala for the Congressional Black Caucus to meeting with key figures in the "Black Lives Matter" movement earlier this month. She is expected to roll out a comprehensive set of policies on criminal justice reform next week.

"Our next president must focus on all of our communities, big and small, and I know that Hillary’s plans for rural communities will help cities and towns like Manning. Her plan to create an infrastructure bank will provide much-needed investments and ensure we have water, sewer and broadband systems," said Mayor Julia Nelson of Manning, South Carolina, in a statement distributed by the Clinton campaign.

Several of the mayors are from South Carolina, the first state in the Democratic primary process with a large black population and a critical one for Clinton, because she is heavily-favored there. Iowa and New Hampshire both have largely white, liberal electorates, and Sanders could win both states.

To be sure, some prominent African-American mayors have not yet backed Clinton, including Muriel Bowser of Washington, D.C. And among mayors who are not black, New York's Bill de Blasio and Los Angeles' Eric Garcetti, who run America's two largest cities, have also not endorsed the Democratic front-runner.