This graphic from Nate Silver is interesting, but I don’t agree with its title:

Yes, most candidates are more unpopular than popular, but I think the story is that Hillary Clinton is more well-known AND more popular than all her possible GOP opponents… with the exception of Dr. Carson who is really unknown and who isn’t going to run anyway.  I guess the good news for Walker and Rubio (the latter who got in the race today officially) is that they are so unknown that they have room to bring up their popularity as people get to know them.  It might be harder for Jeb Bush to that.