New York City Mayor Bill De Blasio (D) urged Sen. Elizabeth Warren (D-Mass.) to endorse Sen. Bernie Sanders's (I-Vt.) presidential campaign Tuesday, claiming Sanders would have done the same if the situation were reversed.

Warren said she would not immediately be endorsing a candidate when she ended her campaign last week.

"I deeply respect @ewarren. Our nation + our party is better + more progressive because of her leadership. Now our progressive movement needs her more than ever," de Blasio tweeted Tuesday. "Senator, if the shoe were on the other foot @BernieSanders would have already endorsed you. We have."

De Blasio, who ended his own presidential campaign in September, endorsed Sanders last month.

Several other former White House hopefuls have backed former Vice President Joe Biden in what has become a two-person race, including former Rep. Beto O'Rourke (D-Texas), former South Bend, Ind., Mayor Pete Buttigieg, and Sens. Kamala Harris (D-Calif.), Cory Booker (D-N.J.) and Amy Klobuchar (D-Minn.).

Biden has a lead in pledged delegates after a strong finish in South Carolina and last week's 15 Super Tuesday races.

Six more states are holding primary races Tuesday, including the 2020 general election battleground state of Michigan.