Mitch McConnell, the Republican leader in the US Senate, will “look past a load of shit” to endorse Donald Trump for president, a GOP colleague said.

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“He’ll look past a load of shit to improve the path to the majority,” the senator said, speaking anonymously to the Hill. “That’d be the one reason why Mitch would rise above principle and do the politically expedient thing … because he is hell-bent on getting the majority, and he’ll make personal sacrifices for that.”

McConnell is the only top leadership figure among Senate Republicans not to have endorsed Trump in his probable rematch with Joe Biden. He is reportedly negotiating how and when to do so.

John Thune, McConnell’s number two, attracted criticism when he endorsed Trump, who he previously said was “inexcusable” for his attempt to overturn the 2020 election and incitement of the deadly January 6 attack on Congress in 2021.

Like Thune, McConnell voted to acquit Trump at his impeachment trial over January 6. But McConnell did excoriate Trump immediately afterwards.

Calling Trump “practically and morally responsible” for the attack, McConnell said the mob had “been fed wild falsehoods by the most powerful man on Earth because he was angry he lost an election”.

“President Trump is still liable for everything he did while he’s in office,” McConnell added. “He didn’t get away with anything yet.”

But Trump has so far evaded accountability for January 6 – or for other election subversion, retaining classified information and paying hush money to an adult film star, the subjects of 91 criminal charges.

Nor have multimillion-dollar penalties in civil cases involving Trump’s business affairs and a rape allegation a judge called “substantially true” significantly dented his standing.

Having won every primary vote, Trump is on the brink of securing the nomination.

Always brittle, the relationship between Trump and McConnell nonetheless paid off for Republicans when Trump was in the White House, particularly through the installation of three conservatives on the supreme court.

After January 6, though, the relationship fell apart. Trump regularly abused both McConnell and – in racist terms – his wife, Elaine Chao, who was transportation secretary until 7 January 2021, when she resigned.

Trump and McConnell have reportedly not spoken in three years. Last week, referring to legislation passed since Biden entered the White House, Trump told a Fox News town hall: “I don’t know that I can work with him. He gave away trillions of dollars that he didn’t have to, trillions of dollars. He made it very easy for the Democrats.”

Nonetheless, multiple reports suggest an endorsement is being negotiated, fueled by McConnell’s desire to take back the Senate.

Another unnamed Republican told the Hill he did not think an endorsement “would be easy” for McConnell, “but he’s a pragmatist and at the end, he’d rather have a Republican or Republican policies”.

“There would be no other reason” for McConnell to endorse Trump than to boost Republicans’ chances of retaking the chamber, the senator said. “I don’t know if his political mind can overcome the personal [issues] enough. If anybody can do it, he can probably do it.”

Thirty-two of 49 Senate Republicans have endorsed Trump. Holdouts include Joni Ernst of Iowa, the only leadership figure below McConnell not to have bent the knee. Ernst has spoken positively about Nikki Haley, Trump’s last primary opponent, but has also refused to rule out a Trump endorsement.

McConnell has said he will support the eventual Republican nominee. On Tuesday, he declined to comment about speculation that he will soon back Trump.