Trump could try to get a court order to prevent it, and if he did, then Bolton would risk being sanctioned by the court who issued the order if he violated it... but from what I've seen/read many judicial pundits think the courts would decline to issue such an order. And if the courts do decline to get involved thusly, then there is nothing the executive branch can do (legally anyhow) to stop him. If Bolton were still an executive branch employee he could be fired... but since he's already been fired...

Does anyone think Bolton rolling on Trump will prevent him from getting into a new position of power which would otherwise still be available if he remained slavishly loyal to Trump? It seems to me that the answer is "no" at this time, because of how badly Trump is damaging the goals/ideology of Bolton's ilk. Whether Trumpist or Democrat, there is no clear route to Bolton getting his ideology immediately enacted. So submarining Trump is probably on the agenda, but not necessarily in a way which helps Democrats.

Really the only thing Bolton cares about is how crossing Trump to testify in such a situation would affect his own interests (personal/financial and ideological/political). Bolton is not going to help Democrats except by accident or incidentally. Whatever testimony he wishes to deliver is bound to be very, very, very self-serving. Democrats are damned if he does and damned if he does not, tbh.

Schiff said the same thing in his closing remarks about the fucking moron. The same would apply it to every GOP senator or congressman too. The scum of the Earth.

... just older and with a ridiculous mustache.

He is a repug neo-war warhawk, remember, he just wants to personally profit from his book making news so he will not leak out anything important until then. He thinks just "volunteering" to come is enough to save his reputation and book and knows drumpf will claim Executive Privilege so is really just a self-serving lie. He doesn't care enough to stop the PUtin tool from stealing another election - he's a repug so even though dastardly, Bolton won't give up the dirt because it would hurt his party too much and he knows it.

If he was going to save the nation, he'd have already done it.