All countries work against other countries by messing in the other country's politics. The US has done it and still does it more than Russia ever could. So what!
The Dems think that Mueller is trying to bring down the Repub party and Trump too, but that's wrong. Mueller, who is working in the same interests as their FBI, is trying to force Trump out of the American picture and that's for obvious reasons that have to do with Trump and his slow destruction of the establishment norms. Have Trump resign quietly and preferably see the whole thing fizzle out. And failing that, maybe resorting to the Kennedy solution for Trump.
Destruction of a president and his political party are far from the ideals of Mueller and/or the FBI. Holding up the establishment and it's agenda are the goal. If Mueller can convince Trump to step down after his 4 year term, over the 'threat' of taking Trump down and taking down his kids too, then Mueller will have succeeded. It won't be anything for the Dems to celebrate and that's because Trump can make it appear to be his choice.
A win/win for both parties!