Representative James E. Clyburn of South Carolina, a veteran lawmaker who was once the highest-ranking Black member of Congress, will endorse former Gov. Andrew M. Cuomo on Friday.

Mr. Clyburn will make the case that Mr. Cuomo is the right candidate to help Democrats fend off the ill effects of the second Trump term, when the president is “challenging the pillars of our democracy.”

“The mayor of New York is uniquely positioned to play an important role in the future of the national Democratic Party,” Mr. Clyburn said in a statement, adding that Mr. Cuomo had the “experiences, credentials and character to not just serve New York, but also help save the nation.”

The endorsement comes three days after Mr. Cuomo’s main rival, Zohran Mamdani, was endorsed by Senator Bernie Sanders of Vermont, the progressive wing’s longtime standard-bearer. Mr. Cuomo and Mr. Mamdani have led in the polls, and the tenor of the race has become increasingly nasty ahead of the Democratic primary on Tuesday.

Mr. Sanders has also placed the race in a national context, arguing that Mr. Mamdani represents a break from ”corporate-dominated politics driven by billionaires.”