The country that gave 51% of the vote to Barack Obama in 2012 and is currently giving him a 50% approval rating is not going to suddenly turn around and elect his antithesis, Donald Trump, because he (Trump) "speaks his mind".
Obama's popularity doesn't indicate that Hillary will win. Bill Clinton was pushing 60% approval rating in 2000, yet his boy Al Gore still lost the election.
But Clinton wasn't out there hammering Bush and lobbying for Gore, probably because his personal favorables were in the trash. This time around it's the President who has solid personal favorability and job approval, and the candidate (Hillary) that's in trouble. Obama is a great surrogate for Clinton, and most importantly Trump is viewed at least as negatively as Clinton and has no Obama figure out there to praise him and hit Hillary.
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