Biden has another card to play in soliciting Warren’s support. He would be 74 years old if he were elected president in 2016, an issue he may have a way of addressing. “One thing that I keep hearing about Biden is that if he were to declare and say, because age is such a problem for him if he does, ‘I want to be a one-term president. I want to serve for four years, unite Washington. I’ve dealt with the Republicans in Congress all my public life,’” liberal journalist Carl Bernstein told CNN this month.

A one-term pledge by Biden would also interest Elizabeth Warren. She is 66 years old, and if a Democrat wins in 2016, she will be 74 herself by the time someone else has served two terms in office — and facing her own age issues. But if Biden won, after pledging to serve only one term, Warren would be the front-runner for the Democratic nomination in 2020. If Biden made her his vice-presidential choice, as Yale’s Professor Kahn and others have suggested, she might be a virtual lock for the Democratic nomination.