WASHINGTON — As crunch time emerged in the GOP’s push to pass a $1.5 trillion tax cut, wavering Republican senators inevitably made the trip from the Senate floor to an office tucked just a few paces away.

They entered and walked past a painting of Alamo legend William Barrett Travis. Past the crackling flames of one of the Capitol’s few working fireplaces. Past a fake Christmas tree, at least in the latter stages of the proceedings, that’s festooned with lights.

And then they aired their grievances to Texas Sen. John Cornyn, the Senate’s No. 2 Republican.

"There was always two or three things with each of these individuals," he told The Dallas Morning News in that same office that's home to his whip operation. "The implied threat that they would withhold their vote for the final product was enough to require a negotiation.

“And that’s basically how legislation gets written.”

Those delicate deliberations stand at the center of Republicans’ ability this week to pass a sweeping tax revamp for the first time in three decades.

Cornyn is not the only Texas GOP'er who served a critical role in an overhaul that gives the vast majority of Americans a tax cut, while giving the biggest boost to corporations and the wealthy. Rep. Kevin Brady of The Woodlands, for one, is the House’s top tax writer.

But Cornyn's behind-the-scenes work also offers insight into his powerful but sometimes nebulous job as Senate Majority Leader Mitch McConnell’s top deputy.

Sens. Marco Rubio of Florida and Mike Lee of Utah wanted more on the child tax credit. Sen. Susan Collins of Maine wanted to protect more of the state and local break. Sens. Ron Johnson of Wisconsin and Steve Daines of Montana wanted more for “pass-through” businesses.

And all of them got something, inoculating against opposition that could’ve sunk the bill.

“For better or for worse, I was involved in all of those,” Cornyn said.

This objection should be dismissed as the last minute ideological grenade it clearly is--not serious, and even bordering on fake news. https://t.co/nGGeVbagq7 — Senator John Cornyn (@JohnCornyn) December 18, 2017

The nature of some GOP deals have drawn scrutiny, with critics seizing on a report that the Texan helped insert provisions to enrich some Republican lawmakers. One measure involved real estate enterprises, while the other dealt with partnerships often used by pipeline companies.

Other news reports have since cast doubt on the alleged quid pro quo, though the proposals would indeed provide some Republicans a boost. A Cornyn spokesman said the partnership measure, for instance, "simply preserves" the tax status those businesses now have.

And Cornyn ripped the attacks, saying they were the “sort of tactics that would make a Russian intelligence officer proud.”

“That’s the most disappointing thing about the state of social media and some of these outlets posing as legitimate news organizations,” he said. “They make things up out of whole cloth. And they are just not true.”

Here are three other inside takes from Cornyn on what he called the “most challenging piece of legislation I’ve worked on.”

‘So technical, so complex’

The Texan figured he knew “generally about the tax code.” He even took a class on federal income tax back when he was in law school years ago.

“But it is so technical, so complex, and so difficult to deal with,” he said. “It really took a lot of education.”

Republicans learned from their failed effort this year to repeal Obamacare, Cornyn said. So there was a concerted effort to engage individual senators before sticking points turned into the kind of jams that could topple the GOP’s narrow Senate majority, he said.

Cornyn said Johnson, for example, introduced him to the world of pass-through businesses.

Those entities pay taxes on their owners’ personal returns. Some are quite small. Others are massive operations like President Donald Trump’s real estate empire. Cornyn said the challenge was figuring out how to give those businesses a tax cut on par with what corporations got.

Sen. Ted Cruz, R-Texas, arrived as Congress prepared to vote on the biggest reshaping of the U.S. tax code in three decades Tuesday. (Andrew Harnik / The Associated Press)

‘Ted Cruz 2.0’

One dramatic moment in the tax debate came when Tennessee Sen. Bob Corker threatened to scuttle the GOP’s push before a key vote one evening.

Texas Sen. Ted Cruz and other Republicans rebelled against Corker's idea to include some $350 billion in tax increases to address his deficit concerns. And that pushback prevailed, with Cruz telling The News recently that it "was really a battle to see who could persuade our colleagues."

Cornyn said the episode was a good example of how proposed solutions sometimes chased “two or three or more people off of the bill.”

“That’s the balancing act,” he said.

Cornyn also reiterated that his fellow Texan was a great help on the tax bill. While Cruz has sometimes been in a thorn in the side of Senate GOP leadership, Cornyn stressed that his colleague “has been very constructive.”

“I think his office at one point called it Ted Cruz 2.0,” he said. “And I like Ted Cruz 2.0.”

View from White House celebration of #TaxCutsandJobsAct pic.twitter.com/0Hyo6VgdB1 — Senator John Cornyn (@JohnCornyn) December 20, 2017

‘Welcome to reality’

Democrats have blasted the GOP for making the individual tax cuts temporary and the corporate ones permanent. They’ve chided Republicans for jacking up the federal deficit. And they’ve railed against provisions like the repeal of Obamacare’s individual mandate.

Cornyn isn’t moved by those critiques.

“So far, most of the news and arguments about the bill have been negative,” he said, predicting that the bill will rev up the economy. “To me, that’s an argument for the status quo. That’s an argument for stagnant wages and anemic economic growth.”

As for Democrats’ argument that they were left out of the process, the Texan insisted that there were “opportunities for both parties to participate.”

“People may say, ‘We’re in the minority. Our amendments didn’t get accepted,’ ” he added. “Well, welcome to reality. That’s the nature of the process.”