We have a treaty with Ukraine, to mutually investigate crimes by citizens in each country.
Senators Menendez, Durbin, and Leahy wrote a letter to Ukraine regarding the Muller probe - so, Democrats do it too. It's a matter of public record.
https://www.foreign.senate.gov/imo/media/doc/5-4-18%20Menendez%20joint%20letter%20to%20General%20Prosecutor%20of%20Ukraine%20on%20Mueller%20investigation.pdf