I edited my above comment with some links and will just add them here: Edit because apparently people don’t believe me. Googling “DNC mailers voter information” and I was able to pull up plenty of articles, though I do not have the letter I personally received. Please note that I know they are not from the DNC, I was just looking for generic words that google would associate with dems.

minnesotareformer.com/…/why-you-keep-getting-thos…

baltimoresun.com/…/maryland-ag-nonprofit-sending-…

cbsnews.com/…/voter-report-card-mailings-describe…

The second article (mentioning that they received a cease and desist for what the state deemed intimidation) says “The groups described themselves as nonpartisan, though their founder, Page Gardner, and CEO, Tom Lopach, are both former Democratic strategists.”

Since you seem to want specifics, there are pics in the articles, but “The mailers also contain a statement that the center “will be reviewing these records after the election to determine whether or not you joined your neighbors in voting.””