Why wouldn’t donations be a reasonable proxy? 3,000 samples is more than enough to have a statistically significant analysis of a population of 100,000.
It's not a valid sample because it's not a random selection - statistical sampling isn't only about sample size.
One reasonable conclusion that can be drawn from the above numbers is that the Democrats in government are less stingy about donations, or that the Republicans there don't really like the GOP and are there for the money alone.
[0] https://www.opensecrets.org/political-parties
[1] https://www.opensecrets.org/elections-overview/donor-demogra...
I'd expect employees who have been there longer, and so are likely making more money, to be both more likely than shorter term employees to make political donations and to be likely to make bigger donations than those shorter term ones that do donate.