Should Hillary Clinton worry about Joe Biden?

She's staring down the worst polling numbers of her campaign, struggling to overcome the scandal surrounding her private emails and grappling with persistent chatter that Vice President Joe Biden might challenge her.

But Hillary Clinton has one key advantage going into primary season: A firm perception that she's the only Democrat who can keep the White House.

"Except for Hillary who is there?" said Brent Budowsky, a Democratic insider who worked for party luminaries including former Sen. Lloyd Bentsen. "There is really no one else at this moment who has...