President Donald Trump's feud with an outgoing Republican Senator is escalating as the GOP enters a new phase in its tax reform push.

Trump lobbed insults at Senator Bob Corker in several tweets on Tuesday, saying that the Tennessee Republican, who in September announced he won't seek reelection, is "fighting" tax cuts and "couldn't get elected dog catcher" in his home state. He later called him a "lightweight" and "incompetent." The tweets came after Corker in a television interview on Tuesday morning said the White House should "step aside" in the tax writing process.

"Tax writing committees in the Senate and the House are going to be laying out the $4 trillion in loophole closings that need to take place. Hopefully the White House will step aside and let that occur in a normal process," Corker said in an appearance on NBC's Today show. "I know recently, the White House in a couple of cases, has been taking things off the table. I would just say, there's a lot of work to be done. The spinach part of this, the tough part of this, is upcoming over the next several weeks."

The remarks apparently triggered Trump's response on Twitter.

And Corker responded with a tweet jab of his own. "Same untruths from an utterly untruthful president. #AlertTheDaycareStaff," he wrote.

The tweet was an allusion to another Corker tweet earlier this month, where he lamented that the White House has "become an adult day care center."

Corker said in a later interview with CNN that he wouldn't support Trump for election to the presidency again. 

The feud between Trump and Corker that gained steam earlier this month has likely made more than a few Capitol Hill Republicans uncomfortable, especially as lawmakers head to work this week focused on advancing tax reform efforts.

The House of Representatives is slated to vote on Thursday to formally back the Senate's 2018 budget resolution, paving the way for legislation to be passed through the budget reconciliation process that requires only a simple majority. The GOP has not yet drafted a tax bill.

The Senate budget allows for a $1.5 trillion increase in the deficit over a 10-year period. Corker voted in favor of the budget last week but in an interview earlier this month said he wouldn't agree to add "one penny" to the deficit, making how he will ultimately vote on tax legislation unclear.

Trump on Monday tweeted that there would be no change to the amount of pretax income American workers can save in 401(k) retirement accounts, killing an idea the GOP was considering as a revenue raiser. The New York Times reported on Tuesday that Republicans are becoming increasingly concerned the president's off-the-cuff remarks and tweets will make already-difficult tax efforts even harder.

Trump on Tuesday will head to Capitol Hill for a policy lunch with Republican Senators, including Corker.

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