The American political system woke up Tuesday morning to find the Republican president of the United States blasting the Republican chair of the Senate Foreign Relations Committee as a “negative on anything Trump” guy who “couldn’t get elected dog catcher in Tennessee.”

Sen. Bob Corker in turn fired back, condemning “untruths” from an “utterly untruthful president.”

Bob Corker, who helped President O give us the bad Iran Deal & couldn't get elected dog catcher in Tennessee, is now fighting Tax Cuts.... — Donald J. Trump (@realDonaldTrump) October 24, 2017

...Corker dropped out of the race in Tennesse when I refused to endorse him, and now is only negative on anything Trump. Look at his record! — Donald J. Trump (@realDonaldTrump) October 24, 2017

Same untruths from an utterly untruthful president. #AlertTheDaycareStaff — Senator Bob Corker (@SenBobCorker) October 24, 2017

Trump had some final remarks.

Isn't it sad that lightweight Senator Bob Corker, who couldn't get re-elected in the Great State of Tennessee, will now fight Tax Cuts plus! — Donald J. Trump (@realDonaldTrump) October 24, 2017

Then Corker went on CNN to say he regretting supporting him in the first place.

.@SenBobCorker asked if he regrets supporting Trump for president: "I would not do that again." (via @mkraju) — David Wright (@DavidWright_CNN) October 24, 2017

Corker added fuel to the fire of his feud with Trump

The precipitating event appears to be remarks Corker made earlier in the morning to NBC in which he said the White House ought to “step aside” and let the relevant committees in the House and Senate work out the details of a tax bill.

“I think that’s the best way for us to have success” he said, before dismissing Trump’s scheduled appearance at today’s congressional lunches as a “photo op.”

But this was just the latest round in a feud that has been under way for a while, dating at least to Corker’s announcement that he won’t run for reelection in 2018. Freed from the burdens of electoral politics, Corker spoke publicly about his view that Rex Tillerson, James Mattis, and John Kelly are “people that help separate our country from chaos.” He also criticized Trump’s tweets on North Korea and, earlier, criticized Trump’s apologia for white supremacist violence in Charlottesville.

That seems to have inspired Trump to mock “liddle” Bob Corker back on October 8, inducing Corker to fire back that “it’s a shame the White House has become an adult day care center.”

The stakes on tax cuts are high

Beyond the schoolyard taunts here, the key underlying issue is actually tax cuts and the budget deficit.

For months now, Republicans have been veering between the idea of deficit-increasing tax cuts, and some kind of fully paid-for tax reform that would offset lost revenue from cutting rates by closing loopholes and deductions. The framework the White House agreed to with congressional Republicans is ambiguous in that regard, committing to steep rate cuts while gesturing at offsetting revenue raisers with considerably less detail.

Trump, personally, has long-seemed more enamored of the idea of just cutting taxes while Corker has signaled opposition to “adding one penny to the deficit.”

There’s wiggle room here in the form of dynamic scoring — simply asserting that the growth-boosting power of tax cuts will make the deficit go away — but wiggle room only works as well as you want it to. If Corker sincerely opposes deficit-increasing tax cuts, then that’s going to make things very difficult for a GOP tax package, since potential offsets tend to be unpopular. A conventional approach to legislating would hold that you catch more flies with honey and Trump should be courting Corker rather than smack-talking at him. But conventional has never been Trump’s approach to politics.

Lunch today with the Senate GOP caucus should be interesting.