I don't think this proves any trend until it is shown that in national presidential elections the polls are off.
Leftist pollsters accurately predicting leftist election wins do not validate their models, it only validates their prejudices. Only multiple examples of their models accurately predicting wins by ideological opponents can validate their models.
SF Giants fans predicting the Giants would win the 2014 World Series before the season started does not make them brilliant baseball analysts. They got lucky.