A key Republican senator is echoing a long-running Republican talking point about the deficit that could spell trouble for the GOP’s tax cuts plan.

Sen. Bob Corker (R-TN) says he will not vote for a tax plan “adding one penny to the deficit.” Corker, who sits on the Senate Budget Committee and who recently announced his retirement from the Senate in 2018, told CBS’s Chuck Todd Sunday that he will stay true to his deficit hawkishness — even though the party seems to be signaling that the deficit should not stand in the way of slashing individual and corporate tax rates.

“If it looks like to me, Chuck, that we are adding one penny to the deficit, I’m not going to be for it. Sorry, it is the greatest threat to our nation,” Corker said.

Republicans are eager to move on to their tax bill after failing to pass an Obamacare repeal-and-replace bill — the party is unified in its goal to cut taxes, but is locked in an intraparty struggle over whether to pay for those cuts.

Corker’s comments are a major signal that Republicans will be forced to answer difficult questions about the deficit in the months ahead. As we saw with the health care bill, Republicans have few votes to spare to reach the necessary 50 to pass a bill through reconciliation.

Already a preliminary nonpartisan analysis from the Tax Policy Center has estimated the Republicans’ tax framework would likely reduce federal revenue by $5.6 trillion over the next 20 years — a tough reality for the many Republicans who ran on a tough-on-spending, reduce-the-deficit platform to win office.

“Everyone was a fiscal hawk — kind of, not really but kind of — up until the election,” Corker said. “Now it seems like there is a party going on up here; to heck with revenues, to heck with constraining [budgets].”

The Senate Budget Committee released a budget proposal last week that allowed for the tax reform package to increase the deficit by $1.5 trillion over the next 10 years — a deal Corker struck with Sen. Pat Toomey (R-PA), who sits on the Senate’s chief tax-bill-writing finance committee.

Conservatives and fiscal hawks are already trying to reconcile the potential of blowing a hole in the deficit by forecasting huge economic growth numbers — claiming the tax cuts will lead to GDP growth double what projections from the Congressional Budget Office say. Though Corker is open to this fuzzy math, he seems to be telling leadership he won’t be an easy vote.

“I’m willing to accept a reasonable score on dynamic growth,” he said of the rosy economic projections used to sell tax cuts.

He didn’t say what he would consider “reasonable.”