To mount his defense at his Senate impeachment trial, Donald Trump will dispatch a bespoke legal team. But the president will also rely on some help from inside the room.

The majority leader, Mitch McConnell, who has promised a thumb on the scale for Trump, would seem to be the best friend any allegedly criminal president could ask for.

But the senator who has emerged as Trump’s impeachment bulldog by protesting the president’s innocence most loudly is South Carolina’s Lindsey Graham.

“This thing will come to the Senate, and it will die quickly, and I will do everything I can to make it die quickly,” Graham said last month. “I am trying to give a pretty clear signal I have made up my mind. I’m not trying to pretend to be a fair juror here.”

Much has been made of how Trump brought the Republican party to heel. But in no single member of Congress has the transformation from Trump skeptic to true believer been so remarkable as in Graham.

About 20 Republican defections would be required to convict Trump in the Senate and remove him from office. No one is predicting that will happen, but Trump could still sustain political damage if the trial features new witness testimony or other evidence about his alleged wrongdoings.

Graham has placed himself at the front of the coalition of Trump defenders to ensure that will not happen. It’s a long way from where he used to stand.

The senator was once an open enemy of the president, decrying Trump’s attacks on his friend John McCain, the late Arizona senator, and tweeting in May 2016: “If we nominate Trump, we will get destroyed.......and we will deserve it.” Graham did not vote for Trump that year, instead casting his ballot for the independent candidate Evan McMullin.

But the political gravity of Trump’s victory soon bent Graham in his direction. As the pair became golfing buddies, the emotional temperature of Graham’s tweets about the president rose. Then came the moment that wholly redeemed Graham in the eyes of the White House.

In public confirmation hearings for the supreme court nominee Brett Kavanaugh, who stood accused of sexual assault, Graham broke with the subdued tone of Republican colleagues to angrily attack Democrats.

“What you want to do is destroy this guy’s life, hold this seat open, and hope you win in 2020,” Graham told Democrats.

By the time impeachment rolled around, there was seemingly nothing Trump could say that Graham would not enthusiastically amplify. When Trump called the impeachment inquiry a “lynching”, Graham said: “This is a lynching in every sense. This is un-American.”

Graham’s transformation has fed a cottage industry of think pieces. But the short version notes that Graham, 64, who has served in Congress for nearly three decades, is up for re-election this year in a state Trump won by 14 points and where the president remains very popular.

Had Graham lost the faith – and endorsement – of Trump this fall, he could have been vulnerable to a Republican challenger in a Senate primary contest. And having Trump on his side won’t hurt Graham in the general election, when he will face a former head of the Democratic National Committee.

If there is a line Graham will not cross in his cultivation of Trump’s favor, no one else can see it. In November, Graham requested documents for a judiciary committee inquiry into the activities in Ukraine of Hunter Biden, former vice-president Joe Biden’s son. Republicans have sought to deflect criticism of Trump by pursuing conspiracy theories about the Bidens advanced by the president.

The elder Biden is not only a former Senate colleague of Graham but a beloved friend. In an on-camera interview a teary-eyed Graham once called Biden “as good a man as God ever created”.

Asked in November about Graham’s now impugning his son, Biden chalked it up to politics.

“They have him under their thumb right now,” he said of the Trump administration’s hold on Graham’s political future.

“I am disappointed, and quite frankly I’m angered by the fact – he knows me,” Biden said. “He knows my son. He knows there’s nothing to this.”

In the House, Trump had a clamorous kennel of bulldog defenders, representatives like Jim Jordan and Devin Nunes, who could turn any discussion of alleged extortion by Trump into an outraged harangue about the sins of Hunter Biden.

The Senate dynamic will be different. Instead of making speeches, the senators must sit and listen and may only submit questions in writing. The deliberation that would precede a final vote has by precedent unfolded behind closed doors and out of the public eye.

Graham is uniquely experienced for the task at hand, having served as an impeachment manager when he was in the House and Bill Clinton was in the congressional crosshairs. At the time, he argued that hearing from witnesses and taking evidence were essential to a fair hearing.

As recently as October, Graham seemed like a potential member of the group of moderate Republican senators who might entertain the idea that Trump had acted improperly.

Asked by Capitol Hill reporters whether anything could sway him on impeachment, Graham said: “Sure. Show me something that is a crime. If you could show me that Trump actually was engaging in a quid pro quo, outside the phone call, that would be very disturbing.”

Graham now says the Senate should call no witnesses and move to dismiss the impeachment trial as quickly as possible.

“Lindsey is about to go down in a way that I think he’s going to regret his whole life,” Biden said. “I say Lindsey, I’m just embarrassed by what you’re doing. For you. I mean, my Lord.”