That doesn't scale well for a country the size of the US, where local voting is determined by residency. For national elections, sure, no registration would be fine. But how would you keep people from voting for elections outside of their region?
This is not really a hard problem to solve as is evidenced by literally every other country. Moreover, in almost every country it is possible to vote even outside of the country.
I can't comprehend why people try to insist that ID is racist and impossible to implement. It is so bizzare my only explanation is propaganda.
The US, unlike many other countries, doesn't have a national ID card issued to citizens. So to obtain an ID, citizens need to usually have either a passport, which costs $85, or a certified birth certificate, which can cost up to $45. And ironically, you often need valid ID to request a birth certificate.
So for lower income citizens, the cost can be prohibitive. This then becomes the equivalent of a poll tax, which historically was used to disenfranchise minorities.
So I'm not saying it's impossible to implement. If the Federal government wants to start issuing free national ID cards, then that would be acceptable in my opinion. However, this would trigger a huge part of the population that doesn't want government tracking etc.
