What goes around . . .

In what parallel universe is it OK for the president of the United States to mock and belittle the chair of the U.S. Senate Foreign Relations Committee from his own party?

Oh sure, Tennessee Sen. Bob Corker, who having announced his intention not to seek reelection, is taking advantage of that blissful state of freedom to speak his mind about Donald Trump — both in an interview with The New York Times and using Trump’s own weapon of choice, Twitter.

But it was Trump who touched off the latest brouhaha with a Sunday morning tweet accusing Corker of not running for reelection because he “didn’t have the guts.” Corker responded with a tweet of his own, saying, “It’s a shame the White House has become an adult day care center. Someone obviously missed their shift this morning.”

The senator told the Times he feared Trump was so reckless in his behavior that he could set the nation “on the path to World War III.” Corker also spoke about the constant effort by key White House staffers “of trying to contain him [Trump].”

On Tuesday Trump was still tweeting: “The Failing @nytimes set Liddle’ Bob Corker up by recording his conversation. Was made to sound a fool, and that’s what I am dealing with.”

Both a Corker staffer and the Times — with Corker’s permission — recorded the phone interview.

But for what earthly reason would Trump escalate a fight with a man whose vote he desperately needs — on tax reform, for any possible action on Iran sanctions, for all diplomatic post confirmations including any possible replacement should Secretary of State Rex Tillerson decide to make an exit from what Trump has made one of the more thankless Cabinet posts.

Surely as a businessman, Trump learned the basic rule that what goes around comes around. And right now Donald Trump needs “Liddle” Bob Corker way more than Corker needs him.