Their credibility is always going to be based on their last poll before the election. They can always claim the current polls were right, but the public’s mood shifted right before the election.
There’s also the conspiracy theory angle where most of the polls sponsored by MSM media outlets may be trying to show a Clinton win. Because the MSM both hates Trump more than McCain and Romney and believes Clinton is more at risk of losing and needs more help than Obama in 2012.
I don’t even know what all the polls said 2 weeks before the vote in 2012. It’s hard to go back and look at more than that final poll before the election from each outfit.
It may not be worth spending time looking at the polls so much right now anyway, because of the fact things can change rapidly. I’m always concerned about the Dems turning up those “people who don’t pay attention to politics” at the last minute and scaring them into voting against the mean, evil Republican.
The best I’ve found on that point (accuracy leading up to the election) above.