On a federal level, there really isn’t much they can do that I’m aware of. It’d be nice to see them blanket oppose his cabinet picks but that would only be symbolic since they don’t have the numbers. They can’t get any laws passed, and even if they could, Trump has no problem disregarding the law, which the Supreme Court says is fine.

If they wanted to go outside the law I guess they could try to organize some kind of coup, but failure at any stage would not only be career suicide but possibly actual suicide given that treason is punishable by death, and this administration would be gleeful to carry out that sentence against Dems. And if it did succeed, that still wouldn’t do anything about the oligarchy propping up Trump, they’d just have another puppet (Vance) to fall back on.