Hillary never lead by 20 points. Her biggest lead was by 7.1 points on 18 Oct...
James Comey's letter to Congress was October 28, 2016. Coincidence? I think not.
I've always been of the opinion that the e-mail scandal was already baked in. Although I wouldn't rule it out. Hillary's 7.1 point lead had already begun to fall prior to the letters release had already fallen to 3.9 on the 28th of October. The trend was already favoring Trump.
It's interesting that on the 28th you still had 12% of the electorate either undecided or saying they were going to vote third party, Johnson and or Stein. that was 7% saying voting third party, 5% still undecided. So it's possible it did influence that small portion what was still undecided. There's not enough information available to either confirm that or to deny it.
I have some national figures about those who made up their minds in the last week, but they're basically useless since the election was decided in Pennsylvania, Wisconsin and Michigan. Nationally it was 43% Clinton, 43% Trump 14% third party or no answer.
In my state data, Wisconsin only has for the last month, useless for figuring out the last week or since 28 Oct. Pennsylvania has a bunch of N/A's for the last week and Michigan is the lone exception. Those voters who made up their minds in the last week in Michigan went to Trump 50-39 over Clinton with the rest voting third party.
Bottom line, there isn't enough hard evidence or numbers available to either confirm or deny the effect of Comey's letter. Trump could have lost Michigan and still won in the electoral college, so Michigan is useless. The big state is Pennsylvania with all its N/A's. If you go by the existing Trend and the national numbers, one would have to say no. If one goes by Michigan, then the answer is yes. So it's an unknown factor either way or no factor at all. I can't help you.
It's high past time that we start electing Americans to congress and the presidency who put America first instead of their political party. For way too long we have been electing Republicans and Democrats who happen to be Americans instead of Americans who happen to be Republicans and Democrats.