Hard to understand why Democrats aren't pushing harder for Bolton to testify. I get they want it to move quickly, but the guy was heavily involved in these actions, clearly was against them, has a flair for the dramatic, is selling a book, and cannot be "impeached" by the GOP as being a "Never Trumper." If they really want to win, they will need Bolton, and he is right there, dangling fruit, and saying he will testify if he has to comply with a subpoena. There is enough testimony from others that there is no way Bolton pulls the rug out from under the main story. Do what is needed to expedite the subpoena debate in the courts, and if he has to testify get him to hammer the final nail. If he gets insulated from a Republican-leaning Supreme Court, just add it to the obstruction charges.
Win-win.