Vice President Joe Biden has made it quite clear that he's interested in running for president in 2016.  There's one problem with that plan and her name is Hillary Clinton.

Hillary Clinton. Also, Joe Biden.

Clinton is the biggest (non-incumbent) favorite for one party's presidential nomination in modern political history. Why? Because she is a revered figure within the Democratic party -- perhaps even more so than when she first ran for president in 2008.  Check out these new numbers from Gallup.

These numbers show that Clinton is beloved within the Democratic base.  Biden, on the other hand, is be-liked. And beloved beats be-liked every time.

And Biden, who has spent his entire life -- basically -- in politics knows that beating someone who is a) universally known and b) universally loved is very difficult.

Time can change things, of course. Clinton started as a very strong frontrunner in 2008 -- and we know how that ended up.  And, even the most loyal Clinton ally would admit that the second she announces for president (or even indicates interest) she would start to be seen as a politician (again) and her numbers would drop.

Still, she starts at a very high point. And, Biden (and anyone else) will have to make a go/no-go decision -- or, at a minimum, begin to organize for such a bid -- well before Clinton makes her own announcement either way and, therefore, before her numbers begin to fade in any meaningful way.

That fact is why a Clinton-Biden primary is extremely unlikely.