(CNN) On Tuesday, Attorney General Jeff Sessions was speaking to a crowd of young conservatives when a "lock her up!" chant broke out -- a reference to the alleged crimes of 2016 Democratic presidential nominee Hillary Clinton. "Lock her up," Sessions said, laughing. "I heard that a long time over the last campaign."

Ha Ha Ha ... oh wait, this is the top law enforcement officer of the United States joking about imprisoning a former secretary of state (and first lady), despite the fact that no actual charges have been filed against Clinton. And by the way, Sessions authorized the top federal prosecutor in Utah to look into whether the FBI didn't look deeply enough into Cinton's actions related to an energy company named Uranium One.

This would be like the commissioner of Major League Baseball authorizing an investigation into alleged steroid use on the New York Yankees and then being caught on camera at Fenway Park with a Red Sox jersey on chanting "Yankees suck!"

Why is Sessions doing something that quite obviously politicizes the office of attorney general and raises questions about his own ability to be objective? Because he knows that it will make the one person who truly matters to his professional life -- Donald John Trump -- very, very happy.

The origin of the "lock her up!" chant appears to be the Republican National Convention in which then-New Jersey Gov. Chris Christie -- he was on Trump's good side way back then -- asked the crowd from the dais whether Clinton was "guilty" or "not guilty." "Lock her up! Lock her up!" was the response from assembled mass.

Trump, as he always does, seized on that response and turned his own rhetoric on Clinton up to 11.

"And I'll tell you what," he said at an October presidential debate. "I didn't think I'd say this, but I'm going to say it, and I hate to say it. But if I win, I am going to instruct my attorney general to get a special prosecutor to look into your situation, because there has never been so many lies, so much deception. There has never been anything like it, and we're going to have a special prosecutor."

In the closing days of the campaign, chants of "lock her up!" were a regular feature of Trump rallies, fitting right alongside Trump's call and response schtick about Mexico paying for the border wall. It was a perfect encapsulation of the dislike and doubt that Trump supporters carried for Clinton: They weren't totally sure what laws Clinton had broken but they were absolutely convinced that she -- and he husband -- had done something wrong. And that Trump, unlike all of the rest of the politicians out there, would actually do something about it. He wasn't an insider and wasn't afraid to break up the little political club that ruled Washington for decades.

As a piece of political rhetoric then, "lock her up!" was brutally effective. The problem here is that the rules dictated a campaign -- such as they are -- aren't the same rules that apply to governance. Being a candidate for president is different than being president. In the former, you can say just about anything in service of victory; in the latter, there are laws and procedures that attempt to preserve the idea that the President and his Cabinet are leaders of all the people, not just the ones who voted for him.

Trump seemed to bow to that difference two weeks after he won the 2016 election, backing away from his pledge to prosecute Clinton. "I don't want to hurt the Clintons, I really don't," Trump told The New York Times . "She went through a lot and suffered greatly in many different ways."

But as we've seen time and again during Trump's first 18 months in office, what he says one day doesn't indicate what he will say the following day or in a week or a month. And as Trump's time in the White House has worn on -- and as he has struggled to expand his appeal beyond his political base -- he has, again, turned on Clinton and urged Sessions to be far more aggressive in his attempts to get to the bottom of any wrongdoing committed by the former secretary of state.

"Attorney General Jeff Sessions has taken a VERY weak position on Hillary Clinton crimes (where are E-mails & DNC server) & Intel leakers!" Trump tweeted last July. At a political rally in Alabama last fall, as the crowd chanted "Lock her up!", Trump said flatly : "You've got to speak to Jeff Sessions about that."

There's no mystery -- for Sessions or anyone else -- what Trump wants out of the Department of Justice: Less Russia, MUCH more Clinton. So, when a "lock her up!" chant breaks out, Sessions kind of, sort of joins in -- because he knows it's the sort of thing Trump would like. And Sessions, the most beleaguered of the current Trump Cabinet, needs a win with Trump wherever he can find one.

Of course, Sessions isn't just a campaign surrogate for Trump. Or even a US senator from Alabama and former colleague of Hillary Clinton. He is the top Justice Department official, leading a wing of government that the President has repeatedly attacked and sought to undermine. Which makes it inappropriate, at the very least, for him to ever utter the words "lock her up!"

CORRECTION: This piece's headline has been updated to correctly reflect that Sessions only appears to have said "lock her up" once on record -- at Tuesday's event.