[epaulson]

Very. Expensive, and many states have legal restrictions on what you can use it for and who can get it.

Nationbuilder, a sorta-competitor to NGP, has put together a national voter file and it's reasonably priced. https://elections.nationbuilder.com/about/faq

The DNC/NGP voter file, however, is significantly enhanced - for one, it's got a lot of phone numbers, which most states don't include in their lists. There's a lot of other survey and consumer data associated.

For a national voter file that's good enough to use for say a City Council race anywhere in the country, where all you want to know is "who is likely to vote in this non-presidential election", it's doable but very expensive. For anything serious, it's pretty much outside of capabilities of anyone but the parties and some of the very big SuperPACs/very big orgs.