I found this YouGov Poll very interesting. Instead of asking who the respondents support for the presidency, they asked them who they thought could win the presidency if nominated by their party. The survey was done between August 28th and September 1st of a random national sample of 1,000 adults, and has a margin of error of about 4 points.
The general public only sees four candidates as having the ability to win if nominated -- Hillary Clinton (65%), Joe Biden (59%), Jeb Bush (53%), and Donald Trump (52%). About 40% thought Bernie Sanders could win -- and all other candidates were at or below 37%.
A majority of Democrats only see three candidates as being able to win -- Clinton (84%), Biden (70%), and Bush (55%). They were split on Sanders chances, with 48% saying he could win.
The Republicans were a bit more generous, giving seven candidates a chance to win if nominated -- Trump (75%), Rubio (58%), Carson (57%), Bush (55%), Biden (55%), Clinton (53%), and Cruz (51%).
While I found the opinions of the general public (top chart) interesting, the opinions of party members (bottom charts) are probably far more important -- especially for their own candidates. It is highly unlikely that a candidate could win in the party primaries if the members of that party don't think the candidate could win the general election.
