Hmm, I had my first custom race elected with a solid majority of both minors and majors, and in a later game, literally everyone voted for me.
The entire galaxy.
In my latest game, I defied the UP, suffered no setbacks, and proceeded to absorb everyone nearby. I'm currently in control of about 45% of an excessive map, without the UP bonus.
I have no idea who is the leader of the UP at the moment, that might be interesting to check.
I find it mildly annoying my trusted, cherished allies are devoured by my culture.
Apparently in this universe, there's no such thing as friendly, longstanding relationships.