The biggest reason Germans have for re-electing Angela Merkel is the electoral disaster in America.  Germans can read.  Unlike the 60 million American GOP voters, Germans are generally not stupid.
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That being said, it would not be surprising for the AfD to do better than any of the polls indicate.  Fringe parties built on prejudice often do better than expected in elections where there is too much boredom and complacency in the center.
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In about three hours we'll see what happens.