Obama said both Sanders and Clinton are good people.
A new poll puts Clinton up by 17 points over Sanders in New York.
Clinton has a 6-point lead over Sanders, 49 to 43 percent.
Rev. Horace Sheffield says he'll run against the longtime Democratic congressman.
Manan Trivedi, who was the Democratic nominee in 2010 and 2012, will announce a third bid.
Clinton is far ahead among black voters, who make up a big portion of the electorate.
Trump and Clinton are up by significant margins in their respective Maryland primaries.
Her lead is helped by widespread support from black voters.
Democracy for America's endorsement may help him hold a big fundraising edge.