Donald Trump has two major problems: He's guilty of extorting Ukrainian leadership, and nearly all of his closest advisers who are central to Trump's Ukraine scandal are highly unlikely to take a bullet for him. Sure, they're loyalists—up to a point. But all of them had reputations that preceded Trump, and far more governmental experience than he does. Some have ambitions beyond Trump, and most of them believe they're the smartest ones in the room.
Take Attorney General William Barr, for example, who has absolutely run interference for Trump on things like the Mueller report release and the now-pending investigations into the origins of the Mueller probe. Barr has an "executive power" agenda and he's an outright conspiracy theorist, but he also knows the law and the inner workings of the Justice Department and any time he has somehow been implicated in Rudy Giuliani's scheme to extort Ukraine, he has quickly tried to distance himself and his department. After the July 25 call transcript came out, Barr immediately expressed his "surprise" at being tied to Giuliani's efforts. The Justice Department also took immediate exception to White House chief of staff Mick Mulvaney's suggestion that military aid for Ukraine was being held up in part due to Ukraine's willingness to cooperate with an ongoing department inquiry.
To date, the Justice Department has only explored one potential criminal violation related to Trump's actions on the Ukraine call: campaign finance. The legal rationale advanced in support of not further pursuing that criminal inquiry was entirely suspect, but there's clearly a reason Barr's Justice Department didn't also explore bribery, extortion, or other potential crimes. Barr will definitely skirt the edges for Trump, but involving himself in Trump's criminality is where he will draw the line. None of this is to say that Barr's hands aren't dirty or that he isn't willing to go to bat for Trump. But when it comes down to getting ensnared in Trump's illegal dealings, Barr isn't going to willingly allow Trump's stupidity to bring him down. Thus, his reported refusal to hold a press conference claiming Trump did nothing illegal on the call.
The same is true, and perhaps even more so, for former Trump national security adviser John Bolton, who reportedly said he wasn't going to be party to any sort of "drug deal" regarding U.S. foreign policy with Ukraine, called Giuliani a "hand grenade," and instructed his deputy Fiona Hill to take concerns about extortion of Ukraine to White House lawyers. It's not that Bolton is a hero, it's that he ain't going down for Trump—someone he surely looks down upon. In fact, Bolton seems willing enough to tell his story as long as has a legal fig leaf from the courts to do so. After all, Bolton needs to work in conservative circles moving forward, and can't be viewed as having eagerly stabbed Trump in the back.
Secretary of State Mike Pompeo clearly hopes to run for higher office: perhaps the U.S. Senate, and then the presidency. He's not jumping in front of any bullets for Trump if he doesn't have to, but his problem is that he very well may be in too deep to escape getting pulled down by Trump. He was on the call, he clearly knew exactly what was happening, and he stood by and watched as his own diplomats got undermined, smeared, and even removed as they tried to advance U.S. interests abroad. Pompeo always viewed the Trump administration as a means to an end—a resumé booster—but not an end in and of itself. But Pompeo simply cultivated too much access/exposure to Trump not to get burned by him. He may figure he has to go all in for Trump at this point—but out of self-preservation, not loyalty.
And then there's poor "let Trump be Trump" Mick Mulvaney. Mulvaney is the most likely to take a bullet for Trump without even knowing he's taking it. He clearly thinks he's smarter than he is. If he ever had a moral compass, he tossed it along with any guiding principles he once declared as a lawmaker. And he's desperate for praise for his work as chief of staff, among other administration roles he has held. Mulvaney was privy to Trump's entire Ukraine scheme and was reportedly involved with issuing the "hold" on military aid to the country. Whereas Pompeo most likely thought he was playing his role in the Trump administration smarter than he did, Mulvaney simply wants to be viewed as Trump's in-control, right-hand man. In fact, he may have already taken a bullet.
And then there's Rudy Giuliani, who has finally realized he is facing the prospect of criminal wrong-doing. Ever since serving as U.S. Attorney for the Southern District of New York and then mayor of New York City, Giuliani has viewed himself as a principal, not an underling of Trump—or anyone else, for that matter. Giuliani probably figures he could have and should have been president and surely would have done the job better than Trump. There's no way Giuliani is a loyalist in the sense of falling on his sword in order to protect his leader. As others have noted, when Giuliani goes on Fox News and waves his phone around, it's a reminder to Trump and perhaps others that all the fire power in that phone could explode in their faces. And if Giuliani was faced with the prospect of saving himself or saving Trump, he would save himself in a heartbeat. As kooky as Giuliani has gotten, he's still looking out for No. 1, and he'll cut Trump loose in a New York minute if he thinks it's to his benefit to do so.
Trump’s two most dangerous top/former advisers are likely Bolton in terms of impeachment/removal (partly because he still holds sway with GOP senators, who also don’t like Trump) and Giuliani in terms of criminal exposure. If Trump somehow loses the incentive of his pardon power, Giuliani won’t blink if it means saving his own hide.