Protest loudly. The government has many instances of getting involved in "states rights".
And congressmen and a senator did show up at the courtroom and sat behind Trump. You don't think that has an effect - congressmen in the courthouse objecting to the proceeding? That, itself, is much more impactful for the overall social process of the trial than sitting in Washington wringing your hands and complaining "It's a state thing. I can't do anything about it."
Look if you think the purpose of our elected officials is to roll over and play dead when there is an outrage like this going on then you should probably run for office as a GOPe.You'd fit right in.
What you just stated is akin to sending “a strongly worded letter.” While it is good that they show their support there’s literally nothing legislatively they can do. If they did, that would go directly against any conservative stance on states rights. Many have already spoken out against the nonsensical case and it still goes forward unabated. That’s literally all they can do.