“There are only a couple of people who could smooth out the tension with Bernie, and one them is Joe Biden,” said one Democratic strategist.

The potential problem, say observers, is that Biden and Clinton might find it difficult to get past the tensions that bubbled up last summer and fall as Clinton’s campaign wavered and supporters of Biden openly pressed him to take her on in a primary.

“Obviously, if Hillary is the nominee, he and President Obama will be behind her 1,000 percent,” said one Democratic donor who supported the Draft Biden effort. “But that doesn’t take away the fact there are some continuing frustrations.”

Democrats loyal to Biden and some of his advisers felt Team Clinton boxed them out of the Democratic race. They argue it is natural for the sitting vice president of a two-term president to consider running for the White House as an heir-apparent candidate.