To be fair, this has happened after past presidential elections. In 2005 and 2013, the number of pure independents also increased.
But this year's uptick swing to pure independents is slightly larger. It's enough of a shift that it could indicate some new erosion in the Republican base.
For the last decade and a half, people have been moving away from being solidly one party or the other. But they've often instead moved to being Republican-leaning or Democrat-leaning independents. Since 2004, independents rose from 29% to 44% of voters but mostly because of the increase in leaners.
The rise of pure independents -- independents who aren't just closet Democrats or Republicans -- has been much more modest. From 2004 to 2016, the percentage of Americans identifying as pure independents has risen and declined a point or two per year, especially in the year after a presidential election.
This year, the number of pure independents rose a little more, a 3 to 4% increase over 2017. At the same time, the percentage of Americans identifying as solid Democrats has been flat, and there's been a slight increase in independents who lean toward Democrats.
So while there has been some erosion in the Republican base, it's not at all clear how much Democrats are really benefiting from it. As Franklin puts it: "There is evidence for some erosion of GOP partisanship in 2017, though I think it is a more modest decline than some would suggest. That has not been accompanied by any substantial Dem gains in 2017."
The rise in pure independents also likely signals that the primaries of 2016 were not an outlier, and voters will continue to self-identify more like free agents, even as their partisanship is increasingly predictable. It's one of the paradoxes of recent years: As partisan polarization increases, so does distrust in institutions. But in the end, a party can no longer decide if voters don't really want to be a part of it.