20 percent is a significant amount of people who were basically voting against the two parties putting forth candidates supporting NAFTA in that election.
Trump has come out against TPP and other free trade deals, and also against expansion of H-1B visa program as well, which appeals to many people across party lines who don't like these sellouts of American jobs that are sorely needed at this time.
Hillary Clinton isn't very clear in her history whether she supports free trade deals or not. She seems very poll driven on that issue, and she hasn't spoken in ages on H-1B visas but in last election came out speaking strongly in support of expanding that program. There is a youtube on line of that if you look for it. So she might lose that 20% voter segment (if it is similarly sized this time) to Trump on this, albeit Trump being a less than appealing candidate for many for other reasons. Many Democrats against TPP and H-1B might stay home in that matchup.
But Bernie Sanders has consistently and strongly spoken out against these free trade deals, and also against guest labor program expansion such as H-1B and H-2B too, so that issue would be a wash for voters, and it would be decided on other issues, which is why many reports have him doing better than Clinton against Trump in the GE.
That's why, if it seems that many of us supported Clinton against the lame investigation of Bengazi which was rather a pointless partisan witch hunt, we still aren't behind her in many other areas, and still have an issue with her judgement in putting email on a privatized email server (which isn't a philosophy that Democrats should support in privatizing government infrastructure like IT that should be used to help make government officials accountable if properly set up. If that infrastructure isn't well set up that had her make this move, she should have been more vocal about a need to fix this problem as a real leader should, not hide that she was doing it, even if many other pols, especially in the Republican administration before Obama's did so themselves a lot.
This whole Bengazi incident feels like a big distraction, perhaps even orchestrated by the corporate media and corporate PTB that want to either have her skewered by them, or have her look good, so that the battle in the GE will be between two candidates that they both control there. If at the start it had not been trying to look at the Bengazi crap, and more just an inquiry (and not expensive investigation) as to why she felt she needed to move email to a private server, then this would be a completely different conversation now.