 They said they have to make the case to the president, who remains convinced of the strength of his campaign, that he cannot win against former President Donald J. Trump. They have to persuade him to believe that another candidate, like Vice President Kamala Harris, could beat Mr. Trump. And they have to assure Mr. Biden that, should he step aside, the process to choose another candidate would be orderly and not devolve into chaos in the Democratic Party.