Sen. Bob Corker (R-TN), who announced his retirement in late September, is one of the Senate’s most influential Republican leaders. He chairs the Foreign Relations Committee, giving him a powerful voice on everything from the Iran nuclear deal to North Korea to America’s response to Russian election meddling.

On Sunday morning President Trump launched into a three-part Twitter rant against Corker, claiming that Corker had begged for Trump’s endorsement for reelection and only retired after it wasn’t forthcoming. (In reality, Trump begged Corker to run again, according to the Associated Press.)

Corker responded on Twitter, saying “the White House has become an adult day care center”:

Later that day, Corker gave an interview to the New York Times, saying that Trump was treating the presidency “like a reality show … like he’s doing The Apprentice or something” and warning that the President’s behavior could set America “on the path to World War III.”

Trump naturally did not like that at all, and retaliated on Tuesday with a new nickname for Corker:

Why Trump is attacking Corker

There are a few possible reasons for the explosion of Trump’s feud with Corker on Sunday. Perhaps the most likely cause was his declaration to reporters, "I think Secretary [of State] Tillerson, Secretary [of Defense] Mattis, and Chief of Staff Kelly are those people that help separate our country from chaos.” The barely concealed implication is that President Trump would not be able to govern at all without having those Cabinet members, the “grown-ups,” in the room to clean up his messes.

.@SenBobCorker: "I think Sec. Tillerson, Sec. Mattis and Chief of Staff Kelly are those people that help separate our country from chaos." pic.twitter.com/NjuRX59s6A — CSPAN (@cspan) October 4, 2017

Corker had also publicly criticized Trump for his frequent tweets on North Korea, especially those that depict Tillerson’s diplomatic overtures to the country as a waste of time.

Contrary to Trump’s contention that he was “responsible” for the Iran nuclear deal, Corker was a prominent opponent of it, and pushed through legislation requiring congressional input before sanctions could be lifted further. But now he opposes ripping up the deal entirely, saying, “You can only tear these things up one time.” That appears to be enough to earn Trump’s ire.

Corker’s heterodoxy hasn’t been limited to foreign affairs, either. In August, Corker criticized Trump’s response to white supremacist violence in Charlottesville, Virginia, saying Trump “has not yet been able to demonstrate the stability, nor some of the competence that he needs to demonstrate in order to be successful. … He has not demonstrated that he understands what has made this nation great and what it is today.”

Then as now, Trump responded to the criticism by attacking Corker on Twitter: “Strange statement by Bob Corker considering that he is constantly asking me whether or not he should run again in '18. Tennessee not happy!”

Despite all that, as recently as September Corker was telling reporters he remained committed to Trump’s agenda, saying, “for people to try to act as if there is daylight between us as a result [of the spat] is just not true.”

This is the worst possible time for Trump to make enemies in the Senate

The fight is quite ill-timed for Trump, given his administration’s latest tax reform push. While hardly a fiscal moderate, and on board for the Obamacare repeal effort, Corker has declared he won’t support the tax reform package the Trump administration and congressional Republicans are putting together if it adds “one penny” to the deficit.

That makes Trump’s decision to antagonize him politically baffling, to say the least. Without Corker’s support for a package, Trump would be down to only 51 Republican senators supporting a tax plan. When you consider that includes relative moderates like Susan Collins (R-ME) and Lisa Murkowski (R-AK), and wild cards like Rand Paul (R-KY), who has criticized the tax reform package already, Trump and Senate Majority Leader Mitch McConnell have very little room to maneuver and can’t afford basically any defections.

Under such conditions, the normal political play would be to avoid antagonizing any potential GOP opponents of the law, to keep them all on board as long as possible. It seems Trump has chosen a somewhat different strategy.

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