President Donald Trump and perhaps his top congressional Republican critic are sparring again, this time with Sen. Jeff Flake’s office disputing the commander in chief’s claim that the Arizona Republican plans to oppose the party’s tax overhaul plan.

Trump started the duo’s latest back-and-forth with a Sunday evening tweet predicting the retiring Flake — whom he mocked by referring to him as “Flake(y)” — will “be a NO on tax cuts because his political career anyway is ‘toast.’”

Losing Flake would put the GOP tax proposal in serious jeopardy of failing, robbing Trump of a year-end legislative victory.

[For Murkowski, Tax Overhaul Isn’t Just Business. It’s Personal]

The president wrote that Flake is “unelectable in the Great State of Arizona (quit race, anemic polls) was caught (purposely) on ‘mike’ saying bad things about your favorite President.”

Sen. Jeff Flake(y), who is unelectable in the Great State of Arizona (quit race, anemic polls) was caught (purposely) on “mike” saying bad things about your favorite President. He’ll be a NO on tax cuts because his political career anyway is “toast.”

— Donald J. Trump (@realDonaldTrump) November 19, 2017

Flake was caught on an open microphone at a Friday event in Mesa, Arizona, saying that if Republicans “become the party of Roy Moore and Donald Trump, we are toast.”

The tweet raised eyebrows in Washington, with congressional observers and reporters firing off their own tweets, noting that the senator had not previously announced how he intends to vote on a tax plan that cleared the Senate Finance Committee late Thursday evening.

Several White House officials had not responded to an inquiry seeking more information about Trump’s prediction, including the basis for his assertion. Several hours after the president’s 6:22 p.m. post Sunday, a Flake aide disputed Trump’s prediction.

“Sen. Flake is still reviewing the tax reform bill on its merits. How he votes on it will have nothing to do with the President,” the aide said.

But the president’s tweet introduces another dramatic twist in Republican efforts to score their first legislative victory since Trump was sworn in on Jan. 20.

That’s because Wisconsin GOP Sen. Ron Johnson has already signalted his opposition unless changes are made to benefit small businesses. If both Flake and Johnson oppose the measure, it would leave no margin for further defections for GOP leaders and the White House. And several other Republicans have expressed skepticism over the measure, including over its projected impact on the federal deficit.

Sen. Roy Blunt of Missouri, a member of the chamber’s GOP leadership team, tried to quell the deficit concerns Sunday on NBC’s “Meet the Press.”

“It doesn’t take much in economic growth to offset both of those numbers. If we get back to the 70-year average of 3.2 percent growth instead of the Obama-[era] average of less than two, we’re up to 3 [percent] already,” he said. “We’re anticipating in this bill 2.6 [percent]. And every tenth of a percent of growth there makes a huge difference in federal revenue.”

[10 Things to Watch as the Tax Bill Moves Forward]

On the same show, White House budget director Mick Mulvaney, a former House GOP deficit hawk, was asked whether the Senate proposal is built on gimmickry to get around chamber rules given Republicans and the White House call it a $1.5 trillion tax cut but several independent estimates project a $2.2 trillion hit on the deficit.

“Yeah, to the extent it’s a gimmick, a lot of this is a gimmick,” Mulvaney admitted. “Obamacare was a gimmick to get through these rules in the Senate.”

“And what you should really be looking at is the policies themselves,” he said, “and we think these are excellent policies.”

Trump’s final Sunday evening tweet reflected what his chief of staff, John F. Kelly, has acknowledged is the former businessman’s frequent impatience with the slow pace of the legislative process. He also tried to put a little pressure on lawmakers while they’re back home hearing from voters.

The president wrote to his 42.1 million Twitter followers that GOP senators are “working very hard” on the tax bill, adding: “Hopefully it will not be long and they do not want to disappoint the American public!”

Republican Senators are working very hard to get Tax Cuts and Tax Reform approved. Hopefully it will not be long and they do not want to disappoint the American public!

— Donald J. Trump (@realDonaldTrump) November 20, 2017

South Carolina Republican Sen. Lindsey Graham, a frequent Trump golf partner, has said his party will be in trouble in the 2018 midterms if it cannot pass a tax bill. That is amplified by the fact that the GOP, given its majority in the House and use of Senate rules, can send Trump a tax bill without a single Democrat voting for it.