“The legacy of that struggle is the reason they are unified behind Thompson, who they know will be a powerful voice for improving public schools, and ending the abuse of stop-and-frisk.”
De Blasio had the support of 36 percent of likely voters overall, according to the poll.
New York City Council Speaker Christine Quinn had 21 percent support, and Thompson had 20 percent, the poll found.
The two civil rights leaders could give Thompson a boost. Both Clyburn and Cleaver are former chairmen of the Congressional Black Caucus.
The Post reported Rep. John Lewis (Ga.) would also stump for Thompson, but Lewis tweeted at the Post late Friday morning, "No one has asked me to endorse anyone in the NYC Mayor’s race and I have no plans to campaign for anyone."
The Democratic primary is Sept. 10.
— Updated at 10:53 a.m.