I’m not going to argue that it’s a positive for the Democrats to have disappointing voter registration numbers in several key states, but I do want to warn against thinking too hard (or not hard enough) about this data. Voter registration numbers are highly sensitive to factors that have little to nothing to do with candidate preference.
For example, in my home state of Pennsylvania, you cannot participate in a primary unless you are registered with that primary’s party, so there was a big spike of Democratic registration in 2008 as the Obama vs. Clinton contest was the only thing on the calendar for about five weeks after Mississippians went to the polls. The Republican contest was nowhere near as compelling in that window of time, so lots of independents and even some Republican mischief-makers went and registered with the Democratic Party so they could participate in deciding whether “Bitter Clinging” Obama would be the left’s nominee. In the end, Clinton won a strong victory but that didn’t prevent Obama from easily carrying the state in the general election against John McCain.
This year, despite a lot of interest in the Clinton/Sanders fight, there was much more media attention on Donald Trump and his battle against the Republican Establishment. His celebrity inspired a lot of people to get involved who don’t typically vote in primaries.
On top of that, there’s been a lot of growth in non-aligned voters in states where people can vote in primaries. And Southern States, like Florida, have long had tons of Republicans who never bothered to drop their Democratic Party registration. We see the same thing in non-battleground states like West Virginia and Kentucky, where political attitudes changed much faster than the registration numbers.
To understand the meaning of registration data, you really need to look at all the peculiarities of each state. Republican registration growth in Pennsylvania is not worrying for Democrats here, but it might be in Florida. The growth of non-aligned voters in North Carolina is probably bad news for the GOP even though it seems to be coming at the expense of the Democrats. That’s because the influx of new non-southern (and returning black) voters isn’t politically in tune with the conservative Republicans in the Tarheel State or with Donald Trump.
Still, overall, it’s better to be the party registering the most new voters.