“My sense is that what we have here is a feedback loop. Does media attention increase a candidate’s standing in the polls? Yes. Does a candidate’s standing in the polls increase media attention? Also yes.

“And everything else which sways both journalists and voters in the same direction just increases the correlation. The media and the public and the candidates are embedded in a system where every part affects every other.”

http://www.niemanlab.org/2016/01/how-much-influence-does-the-media-really-have-over-elections-digging-into-the-data