Joe Biden hasn't even announced his candidacy, but he is many Democrats' second choice after Hillary Clinton – ahead of Sen. Bernie Sanders

Vice President Joe Biden is reportedly considering a presidential bid, and Hillary Clinton’s campaign is showing signs of careful concern. The latest YouGov/Economist poll suggests Sen. Bernie Sanders might also want to watch his back, particularly if more Clinton supporters start looking seriously for an alternative.

Previous polls have shown Biden to be in the best position to sweep up Hillary Clinton supporters if second choices are taken into count. And while Sanders still edges in Biden first-preference support among Democratic registered voters (20% to 25%), if you include second choices Biden’s support grows to more than 50%, while Sanders grows to just 42%. Support for Biden has also grown significantly in recent months. On second preferences, Biden picks up more than twice the share of Hillary Clinton supporters as Sanders (57% to 23%).

When all Democrats are forced to choose between Biden and Sanders (leaving Clinton completely out of the picture) they break for Biden over Sanders by 58% to 42%. Biden is buoyed by broad support among Hispanic and black Democrats, as well as those who are not ideologically liberal. Yet even among whites and liberals, the split between Biden and Sanders supporters is much closer to 50-50, suggesting the Vice President has significant sway even with Sanders’s core constituencies.

It’s possible Democrats could move towards Sanders as they get to know him better. Nine in ten Democrats have a favorable or unfavorable opinion of the Vice President; despite Sanders’s surge in the polls, only three quarters of Democrats have an opinion of him.

However, even among the subset of Democrats who do have an opinion of Sanders (by no means a representative group), voters are split: 51% opt for Biden over Sanders (49%) in a forced choice.

Perceived electability may be key. Among those who know of Sanders, 76% say Biden “could possibly win” the general election, and 11% say he can’t, compared to 66% and 24% for Sanders. Even among Democrats who have a favorable opinion of Sanders, 79% believe Biden can win, versus 74% for Sanders (and 88% for Clinton).

Sanders has been repeatedly underestimated, and Biden hasn't even begun campaigning yet (and his previous two runs went badly). Add to that, Sanders's supporters remain more enthusiastic about his candidacy than Clinton's or Biden's supporters. But the findings show that electability and non-white support remain vulnerabilities for the Sanders campaign, even without the Clinton juggernaut in the picture.

See the Economist/YouGov results

Economist/YouGov poll archives can be found here.