Sen. Bob Corker (R-Tenn.) does a TV news interview about the GOP tax bill on Nov. 28. (J. Scott Applewhite/AP)

Every time talk of cutting taxes has come up in Congress this year, Sen. Bob Corker of Tennessee has warned that legislation cannot add a penny to the federal debt — “the greatest threat to our nation,” in his words.

Yet at each juncture, Corker (R) has supported moving along a tax bill that would blow a $1.5 trillion hole in the budget in the next decade, only offering a vague promise that the issue would be addressed in final legislation.

He’s run out of time. The Senate is planning to vote on its tax bill as soon as Thursday, and Corker is promising it will contain an as-yet unseen measure reversing the tax cut if the GOP’s projections about the bill’s impact on the debt turn out to be too rosy in future years.

The question is what Corker, who recently announced he won’t stand for a third term, will do if the  “trigger” fails to survive final legislation. Already, several key Republicans and influential outside groups like the U.S. Chamber of Commerce have sharply criticized the idea as creating unnecessary uncertainty over future tax rates.

“I don’t want to box myself in. I’m just saying this is very important to me,” Corker said in an interview this week. “Everyone has known from Day 1 this is very important to me.”

Critics say they fear that the senator, who has played a part in many of Congress’s big legislative efforts over the past decade and recently engaged in verbal warfare with President Trump, will capitulate.

“Bob Corker has made a career out of protesting very loudly, and then falling in line with his party’s leadership when it counts,” said Brian Fallon, a former spokesman for Sen. Charles E. Schumer (D-N.Y.), the minority leader.

Fallon argued that Corker, for instance, worked with Democrats on an auto sector bailout and on new regulations for Wall Street, only to abandon the effort later on.

“Being a maverick requires actually voting your conscience on the Senate floor, not just saying the right things on the set of Morning Joe,” Fallon said.

Corker has led a group of Senate Republicans, including Sens. James Lankford of Oklahoma, Jeff Flake of Arizona and Jerry Moran of Kansas, to try to amend the Senate bill to take more seriously the risk that it adds to the debt over the long run. In their plan, a threatened shortfall would trigger rate increases or the elimination of deductions to bring in more revenue and tame the deficit hit.

Intentionally or not, Corker and his fellow deficit hawks have laid bare the compromises most others in their party have been willing to make in search of a victory on taxes after a humiliating year-long legislative drought.

“I came here because of fiscal concerns. I’ve seen no real movement, matter of fact probably negative movement, as it relates to deficit issues,” Corker said. “And what I don’t want to do is be a part of something where I’m knowingly involved in making that situation worse.”

It’s a concern that appears to be shared by precious few in the GOP these days. After harping about the ballooning debt during the Obama administration, Republicans are rushing through a tax bill that will add $1.4 trillion to $1.6 trillion to the deficit over the first decade and perhaps much more beyond that, according to most analyses, particularly if Congress renews expiring tax provisions.

The Trump administration and GOP leaders say that economic growth resulting from passage of the bill will more than make up for the revenue loss from dramatically lower corporate rates and tax cuts that disproportionately benefit the wealthy. But they have yet to offer any proof that such growth will materialize.

Corker says that since GOP leaders and the administration decided to proceed without waiting for Congress’s budget scorekeepers to come up with estimates about the bill’s wide-ranging economic impacts, some kind of mechanism was needed to provide him a comfort level that the bill won’t do more harm than good to the debt.

“I’d love to see tax reform take place. Since we’re skipping several steps along the way, I just want to make sure the revenues are there, and we’re not increasing deficits, and these backstop ideas and trigger ideas are ones that I think can resolve that issue,” Corker said.

“If everybody’s so confident that these revenue projections are going to be met, they shouldn’t be a problem,” he added.

Corker has leverage in the closely divided Senate. With Democrats unanimously opposed to the tax legislation, Majority Leader Mitch McConnell (Ky.) can lose only two GOP votes and still pass the bill with a tiebreaking assist from Vice President Pence.

Corker said Tuesday that he had an “agreement in principle, very strong agreement,” with McConnell to include his deficit-curbing language in the final bill.

McConnell said accommodating every member’s concern is proving challenging.

“It’s a challenging exercise. Think of sitting there with a Rubik’s Cube, trying to get to 50. And we do have a few members who have concerns, and we’re trying to address them,” he said Tuesday. “And we know we would not be able to to go forward until we get 50 people satisfied, and that’s what we’re working on.”

For some longtime Washington observers, Corker has become a rare voice in the GOP still sounding the alarm about the $20 trillion debt.

“There’s a snowballing mind-set that’s taken hold that they must pass a big tax cut, and the political momentum behind it is great,” said Bob Bixby, executive director of the Concord Coalition, a nonpartisan organization that advocates for fiscal responsibility. “And it takes guts to stand in front of an avalanche and say stop.”

Still, Bixby is skeptical that even the “trigger” that Corker is pursuing will make a real difference, since lawmakers could waive it at a later date to avoid broad tax increases. Bixby said Republicans could have chosen to structure the tax plan so it would have no impact on the debt.

“It might look like a face-saving device, a fig leaf,” Bixby said. “If there was a real concern about deficits, they could do this revenue-neutral.”

Corker, 65, is a former construction executive and mayor of Chattanooga who has served in the Senate since 2007.

Corker joined with Sen. Mark R. Warner (D-Va.) to work on a piece of the Dodd-Frank financial overhaul legislation passed under President Barack Obama, involving the liquidation of financial firms, but ended up voting against the legislation. He helped craft a border-security compromise that eased a major immigration bill through the Senate in 2013, although it ended up dying in the House. As Foreign Relations chairman, he pushed for Congress to get a vote on the Iran deal, though Congress ultimately failed to reject the accord.

“Bob Corker is one of the most principled guys in the Senate. I think he will take a hard look at some of these growth assumptions,” Warner said.

After supporting Trump for president last year and initially offering optimistic assessments after Trump assumed office, Corker announced his retirement from the Senate in September and broke decisively with the president soon after. Corker charged that Trump was degrading the office of the presidency and potentially setting the U.S. on the path toward World War III. Trump in turn disparaged Corker over Twitter as “Liddle’ Bob Corker.”

Allies say they don’t expect the spat to have much bearing on how Corker votes on the tax bill, but the legislation has exposed yet another fracture between the two men. Both have a background in construction industries, yet while the brash and impulsive Trump has boasted he’s the “king of debt” and shows no hesitancy in pushing a tax bill that’s not paid for, Corker learned the opposite lesson in his years in business, according to Tom Ingram, a longtime ally and former campaign aide.

“As a developer and an entrepreneur, debt’s frequently the killer if you don’t manage it well,” Ingram said. “And I expect he relates his own personal experience.”