But just what could bring down Biden remains an open question among campaigns. Some hope that his verbal struggles in debates and at campaign events, including recent misstatements in a tale of military heroism, could feed growing concern about nominating a man who would be the oldest U.S. president in history. Many discount the value of current polls, since so many voters have yet to focus on the choice and many are open to changing their minds.

Others have grown increasingly skeptical that direct challenges to his record will change the dynamics of the race. Rather than risk alienating his supporters, they are choosing to build a broader argument about generational or policy change — an implicit contrast to Biden and his central pledge to “restore the soul” of the nation and revisit the popular parts of the Obama administration.

“We can’t look like our message is to just, kind of, turn back the clock and go back to normal,” South Bend, Ind., Mayor Pete Buttigieg said on a campaign swing through Iowa last month. “There’s no ‘again.’ It’s about making sure that the future is better than the past and representing something that’s going to be new and different.”

Some candidates, including former housing and urban development secretary Julián Castro, have begun to more explicitly urge Democrats to move out of the safety of the familiar.