It's just a time tested tactic of discrediting a result you dislike in order to prevent your constituents from becoming apathetic and potentially not voting.
If Republicans actually concede that Nate Silver's results are legitimate and unbiased (if they are), than the party would have a much harder time raising money and getting out the vote.
It's better to frame it as a conspiracy so less people take it seriously.
If you follow much of his writing/tweeting, he doesn't disguise much opinion, which has a liberal slant - but the pure math of his project is pretty hard to argue against.
If Republicans actually concede that Nate Silver's results are legitimate and unbiased (if they are), than the party would have a much harder time raising money and getting out the vote.
It's better to frame it as a conspiracy so less people take it seriously.