It's time to start imagining it.
Mueller's probe is looking ever more dangerous to Trump. Two of his former campaign aides have been indicted, and a third has pleaded guilty on false statement charges. The president has repeatedly complained about the investigation both in public and in private, and some of his allies in conservative media have already been preparing a public justification for the firing of the Special Counsel.
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Plus, there are historical precedents. In 1973, Richard Nixon fired his way through the Justice Department until he could fire the special prosecutor investigating Watergate. And in May 2017, the sitting US president — a guy named Donald Trump — fired the FBI director who was leading an investigation into his own campaign.
So this could really happen. But as we consider the possibility that Trump will move on Mueller, there are a couple of things to keep in mind.
First, it wouldn't be as simple as firing one person. It seems that to get rid of Mueller (without a finding of genuine misconduct), Trump would also have to get rid of Deputy Attorney General Rod Rosenstein — and perhaps several more Justice Department officials, until he finds one willing to carry out his order. Meaning it would get very messy.
Second, the past precedents of the Saturday Night Massacre and the James Comey firing both ended up failing to squelch the respective investigations at play. Both provoked a backlash from the public and the political system, and eventually the existing investigation ended up continuing under new leadership.
So getting rid of Mueller would be enormously costly to Trump, and could well fail to end his legal problems. This is likely why White House aides are currently advising against it.
But it's entirely possible that, whether out of fear of what Mueller will turn up or simple annoyance at the investigation itself, Trump will pull the trigger anyway. And if he does so, the political system will be thrown into a major crisis.