Hart Research Associates is one of the top Democratic polling firms in the country. They aren't ranked on 538, but they are the Democratic half of the Wall Street Journal/NBC poll (Republican half is Public Opinion Strategies).
For what it's worth, I think they are the same "Hart" as Garin-Hart-Young.
The one reason to view this with a skeptical eye is that the motivation here is to obtain favorable polling for LGBT rights issues (and then use those polling results to convince politicians to back LGBT rights progress), not to obtain accurate presidential polling numbers. It's always worth being skeptical of presidential numbers in a poll that is really about something else; they're just less motivated to get the numbers right.
