DHS does say that all states are now Real ID compliant, but it doesn't prove citizenship. Real ID is available to non-citizens, such as green card holders, and people residing in the US on non-permanent visas.There is NO obstacle to proving you are a citizen. In most states it's right there on your drivers license, along with a photo. As a matter of fact I just checked and ALL states along with the territories and DC are now Real ID compliant so not only is there no obstacle, there is NO excuse.
You could try passports, but ownership is relatively low, particularly in the south and midwest.
It's also worth remembering that the US has severe penalties for people who vote illegally. Fines of up to $10,000 and up to five years in prison.