You could also work to make an API out of the state data (it's a a ton of work to aggregate all that), or work to make the site an offline application so that voter registration forms can be created, saved, and later emailed, or even printed and snail mailed to voters. The biggest obstacle with voter registration in the US is that you have to physically mail a sheet of paper. I'd be willing to bet that few unregistered voters probably also have a printer, stamps, and an envelope all ready to go.
Now, one caveat to all this is of course that I'm not sure how much of it is completely kosher. There are lots of regulations on this stuff, obviously.
Another possibility: feature that lets people "pledge" to vote by entering their email address. This could be useful to canvassers on the ground trying to catch busy people passing by without the time to fill out a form at that moment. The app could then email the voter to follow up, as well as alert them when their state's deadline is drawing near.
It's not surprising that each group targets demographics which seek to further their aims.
Despite that, registering more voters is categorically a good thing. We as a society require an informed and engaged electorate. Registering to vote is part of that engagement.
Democrats have their share of right-wingedness, but also cater to marriage equality folk, minorities and progressives.
When Republicans give up on 47% of the electorate, it's a clear sign that, for them, voter suppression is more effective than registration.