The “Joe Biden: 2016 presidential candidate” trial balloon has been aloft for about four days now, and my colleague Nate Silver has already addressed why the vice president has little reason to jump into the race “except as a break-glass-in-case-of-Clinton-emergency candidate.” But here’s some more evidence that Biden’s path to the nomination depends heavily on Hillary Clinton’s campaign collapsing: Biden doesn’t have a clear constituency in the Democratic Party.

Three national polls have been conducted over the last month that included Biden and asked his supporters for their second choice. If Biden’s entry into the race were expected to hurt Clinton more than the other candidates, then we’d expect him to disproportionately earn the votes of Democrats who might otherwise be voting for Clinton. That’s not what’s happening.

CANDIDATE 1ST OVERALL CHOICE BIDEN SUPPORTERS’ 2ND CHOICE Hillary Clinton 57.6% 59.0% Bernie Sanders 16.8 11.9 Joe Biden 10.9 – Jim Webb 1.5 12.9 Martin O’Malley 0.8 5.9 Lincoln Chafee 0.4 6.1

Biden pulls proportionally from all the Democratic candidates. About 59 percent of Biden voters say Clinton is their second choice — about the same overall level of support Clinton earns among the Democratic electorate. And about 12 percent of Biden backers would go to Sen. Bernie Sanders, a smaller percentage than Sanders’s overall level of support.

Of course, this math could change if Biden enters the race. He might be able to win the backing of Democratic bigwigs who are unwilling to support Sanders but would be open to an alternative to Clinton. And with Biden in the race, Clinton’s (along with the other candidates’) numbers would drop.

But Biden does not have an obvious camp of support right now.