If the president of the United States wants his attorney general to investigate how the Obama administration handled a surveillance warrant involving a former campaign aide, he should ask him.

But if President Trump did that, he would draw thundering criticism for essentially ordering a Justice Department investigation of his predecessor.

Instead, Trump is turning to his tried-and-true Twitter technique of taunting Jeff Sessions.

The president’s tweet followed Sessions' decision to have the department's internal watchdog examine the controversy over the FISA warrant for Carter Page—the subject of all that Republican-vs.-Democratic memo sniping.

Trump asked: "Why is A.G. Jeff Sessions asking the Inspector General to investigate potentially massive FISA abuse. Will take forever, has no prosecutorial power and already late with reports on Comey etc. Isn't the I.G. an Obama guy? Why not use Justice Department lawyers? DISGRACEFUL!"

That last word is just remarkable.

As an old Justice reporter, let me pose this question:

How credible would it be if Sessions, a big Senate supporter and surrogate of the Trump campaign, who's recused himself from the Russia probe, was overseeing an investigation of how the Obama DOJ handled a surveillance request against a Trump adviser who had contacts with Russia?

That's why you have an independent inspector general. And that job is generally occupied by career prosecutors, like Michael Horowitz, who has worked in both Republican and Democratic administrations.

Sessions was firm but restrained in a statement, saying, "As long as I am attorney general, I will continue to discharge my duties with integrity and honor, and this department will continue to do its work in a fair and impartial manner according to the law and the Constitution."

Trump has privately bashed and tweet-trashed Sessions before, most notably when he was angry that Sessions had recused himself from the investigation now run by Robert Mueller. Things reached the point that Sessions handed in his resignation letter, which the president refused to accept.

Just last week came this presidential tweet:

"Question: If all of the Russian meddling took place during the Obama Administration, right up to January 20th, why aren't they the subject of the investigation?" Trump tweeted. "Why didn't Obama do something about the meddling? Why aren't Dem crimes under investigation? Ask Jeff Sessions!"

Fox's Brit Hume said of the latest tweet that "this is Trump at his worst. He is asking that the DOJ investigate itself. The inspector general, who has his own staff of lawyers and investigators, at least enjoys a measure of independence from the department. Trump still wants the AG to act his political goalie."

On the other side, Jerry Falwell Jr. tweeted that Sessions "must be part of the Bush/Romney/McCain Republican Establishment. He probably supported @realDonaldTrump early in campaign to hide who he really is. Or he could just be a coward."

Is Trump trying to embarrass Sessions into quitting? He's not a big fan of Rod Rosenstein, who would become acting AG, and the No. 3, Rachel Brand, recently quit. The battle for the Senate to confirm a new DOJ chief would be a drawn-out spectacle.

For the moment, the president has left his attorney general little choice but to defend his department.