American political poll puzzles

The American elections remain curious. Democrats argue that the campaign is 'nationalised' that a general dissatisfaction with the Republicans will impel major gains. Republicans espouse the conventional wisdom that 'all politics is local' and that strong MPs will resist the tide. It is the uniform swing argument again. Democrats cite generic polls (and here) that show them ahead, but if this is the case why are they tied with Republicans in Vermont of all places?