Donald Trump held a press conference on Super Tuesday with campaign manager Corey Lewandowski, fourth from left, onstage with him. Lewandowski grabbed a Breitbart reporter by the arm after a similar press conference on Tuesday. (Reuters/Scott Audette)

This post has been updated.

Donald Trump seemed almost friendly at times during his press conference Tuesday following primary victories in Michigan and Mississippi. He called journalists by name and responded to multiple follow-ups for the better part of an hour.

Then his campaign manager, Corey Lewandowski, roughly grabbed the arm of a Breitbart reporter who tried to ask another question as the candidate made his exit, according to a Washington Post reporter who witnessed the incident.

Call it another wild mood swing from an erratic campaign that appears to be going for a more composed image (as Trump pivots toward the general election) but is simply incapable of complete self-control.

Trump’s attempt to play nice(er) with the media can be traced to Super Tuesday, when after taking a commanding lead in the Republican presidential nominating process he held a press conference in lieu of his customary victory rally. During that session, he was unusually patient with reporters’ questions. At one point, he noted — approvingly — that he had been watching all three major cable news stations, from left-leaning MSNBC to right-leaning Fox News. “See? I’m becoming diplomatic,” he said.

Then, at a Fox News Channel debate two days later, the GOP front-runner went out of his way to be civil toward moderator Megyn Kelly, whom he has ridiculed more than any other journalist. “Nice to be with you, Megyn,” he said in greeting. “You’re looking well.”

When Kelly pressed hard on Trump’s many flip-flops, he attacked the argument that he’s a waffler but restrained himself from, as he often does, bashing the questioner.

Of course, Trump wasn't able to completely changes his spots during the debate. Perhaps the most enduring moment was Trump’s guarantee that “there is no problem” with the size of a certain part of his anatomy — a sophomoric and decidedly un-presidential wisecrack that he just couldn’t resist.

Now comes Tuesday’s run-in with Breitbart reporter Michelle Fields, which might not have featured Trump as the main offender but was clearly a result of the culture he has created in his campaign. The details are a bit hazy, but Breitbart chief executive Larry Solov issued the following statement:

It’s obviously unacceptable that someone crossed a line and made physical contact with our reporter. What Michelle has told us directly is that someone “grabbed her arm” and while she did not see who it was, Ben Terris of The Washington Post told her that it was Corey Lewandowski. If that’s the case, Corey owes Michelle an immediate apology.

Terris confirmed that he saw Lewandowski grab Fields by the arm.

The incident was vaguely reminiscent of a violent altercation at a Trump rally in Radford, Va., last week, where a Secret Service agent grabbed a Time magazine photographer by the neck and took him to the ground. The photographer, Chris Morris, had tried to step outside the “press pen” to take a picture and cursed at the agent when told to stay in place.

Tuesday’s encounter between Lewandowski and Fields was clearly less extreme, but the fact that a top Trump staffer would physically accost a journalist from Breitbart, of all places, highlights the extent of his campaign’s volatility. Breitbart has been a major cheerleader for the billionaire real estate mogul, a role The Washington Post’s Paul Farhi described well when he wrote about the popular conservative new site in January.

Long before rebellious Republicans and the rest of the news media took Trump’s candidacy seriously, Breitbart was championing Trump’s “anti-establishment” message, one that seems to square with Breitbart’s own ethos.

Trump has returned the favor, doling out so many “exclusives” to Breitbart’s relentless Washington political editor, Matthew Boyle, that some have wondered whether Trump and Breitbart are in business together. (They’re not, both sides say.) Nevertheless, Trump clearly holds a special place for Breitbart, which is named for its late founder, the activist and media entrepreneur Andrew Breitbart. When Boyle, 28, asked Trump about his rising poll numbers after a Republican debate last summer, Trump broke into a broad smile and high-fived the young journalist in front of startled onlookers in the post-debate spin room.

If Trump’s campaign can’t maintain an even temper around Breitbart, then it’s hard to imagine how he would keep a level head around anyone else as president.

As I’ve written before, journalists aren’t very sympathetic victims in the eyes of many Trump supporters; we get it. But it’s really not about the media. It’s about Trump’s bullying style — which always rears its head, sooner or later, despite his attempts to suppress it.

Voters need to consider whether his approach to media relations is an acceptable approach to running the country.