The proof of citizenship requirement is for state elections. Federal only voters don’t need to show proof of citizenship 

Bell_curve

 are that it's mostly Republicans impacted (and they may well be citizens, just haven't provided/been asked for documentation). I could see Dems making the same case for the 2016 presidential election in AZ here.

Politics_And_Law

Arizona admits that nearly 100,000 non-citizen 'voters' registered

[Current reports](https://www.phoenixnewtimes.com/news/arizona-voter-registration-glitch-what-happened-20151131) are that it's mostly Republicans impacted (and they may well be citizens, just haven't provided/been asked for documentation). I could see Dems making the same case for the 2016 presidential election in AZ here.