WASHINGTON (NYTIMES) The relationship between President Donald Trump and Senator Mitch McConnell, the majority leader, has disintegrated to the point that they have not spoken to each other in weeks, and McConnell has privately expressed uncertainty that Trump will be able to salvage his administration after a series of summer crises.

What was once an uneasy governing alliance has curdled into a feud of mutual resentment and sometimes outright hostility, complicated by the position of McConnell's wife, Elaine Chao, in Trump's Cabinet, according to more than a dozen people briefed on their imperiled partnership. Angry phone calls and private badmouthing have devolved into open conflict, with the president threatening to oppose Republican senators who cross him, and McConnell mobilising to their defence.

The rupture between Trump and McConnell comes at a highly perilous moment for Republicans, who face a number of urgent deadlines when they return to Washington next month. Congress must approve new spending measures and raise the statutory limit on government borrowing within weeks of reconvening, and Republicans are hoping to push through an elaborate rewrite of the federal tax code. There is scant room for legislative error on any front.

A protracted government shutdown or a default on sovereign debt could be disastrous - for the economy and for the party that controls the White House and both chambers of Congress.

Yet Trump and McConnell are locked in a political cold war. Neither man would comment for this story. Don Stewart, a spokesman for McConnell, noted that the senator and the president had "shared goals", and pointed to "tax reform, infrastructure, funding the government, not defaulting on the debt, passing the defence authorisation bill". Still, the back-and-forth has been dramatic.

In a series of tweets this month, Trump criticised McConnell publicly, then berated him in a phone call that quickly devolved into a profane shouting match.

During the call, which Trump initiated Aug 9 from his New Jersey golf club, the president accused McConnell of bungling the health care issue. He was even more animated about what he intimated was the Senate leader's refusal to protect him from investigations of Russian interference in the 2016 election, according to Republicans briefed on the conversation.

McConnell has fumed over Trump's regular threats against fellow Republicans and criticism of Senate rules, and questioned Trump's understanding of the presidency in a public speech. McConnell has made sharper comments in private, describing Trump as entirely unwilling to learn the basics of governing.

In offhand remarks, McConnell has expressed a sense of bewilderment about where Trump's presidency may be headed, and has mused about whether Trump will be in a position to lead the Republican Party into next year's elections and beyond, according to people who have spoken to him directly.

While maintaining a pose of public reserve, McConnell expressed horror to advisers last week after Trump's comments equating white supremacists in Charlottesville, Virginia, with protesters who rallied against them. Trump's most explosive remarks came at a news conference in Manhattan, where he stood beside Chao. (Chao, deflecting a question about the tensions between her husband and the president she serves, told reporters, "I stand by my man - both of them.") McConnell signalled to business leaders that he was deeply uncomfortable with Trump's comments: Several who resigned advisory roles in the Trump administration contacted McConnell's office after the fact, and were told that McConnell fully understood their choices, three people briefed on the conversations said.

Trump has also continued to badger and threaten McConnell's Senate colleagues, including Senator Jeff Flake of Arizona, whose Republican primary challenger was praised by Trump on Twitter last week.

"Great to see that Dr Kelli Ward is running against Flake Jeff Flake, who is WEAK on borders, crime and a non-factor in Senate. He's toxic!" Trump was set to hold a campaign rally Tuesday night in Phoenix, and Republicans feared he would use the event to savage Flake again.

If he does, senior Republican officials said the party's senators would stand up for their colleague. A Republican "super PAC" aligned with McConnell released a web ad on Tuesday assailing Flake's Republican rival, Kelli Ward, as a fringe-dwelling conspiracy theorist.

"When it comes to the Senate, there's an Article 5 understanding: An attack against one is an attack against all," said Senator Lindsey Graham, who has found himself in Trump's sights many times, invoking the NATO alliance's mutual defence doctrine.

The fury among Senate Republicans toward Trump has been building since last month, even before he lashed out at McConnell. Some of them blame the president for not being able to rally the party around any version of legislation to repeal the Affordable Care Act, accusing him of not knowing even the basics about the policy. Senate Republicans also say strong-arm tactics from the White House backfired, making it harder to cobble together votes and have left bad feelings in the caucus.

When Trump addressed a Boy Scouts jamboree last month in West Virginia, White House aides told Senator Shelley Moore Capito, a Republican from the state whose support was in doubt, that she could only accompany him on Air Force One if she committed to voting for the health care bill. She declined the invitation, noting that she could not commit to voting for a measure she had not seen, according to Republican briefed on the conversation.

Senator Lisa Murkowski of Alaska told colleagues that when Trump's Interior secretary threatened to pull back federal funding for her state, she felt boxed in and unable to vote for the health care bill.

In a show of solidarity, albeit one planned well before Trump took aim at Flake, McConnell will host a US$1,000-per-person (S$1,362) dinner Friday in Kentucky for the Arizona senator, as well as for Senator Dean Heller of Nevada, who is also facing a Trump-inspired primary race next year, and Senator Deb Fischer of Nebraska. Flake is expected to attend the event.

Former Senator Judd Gregg of New Hampshire, a Republican who is close to McConnell, said frustration with Trump was boiling over in the chamber. Gregg blamed the president for undermining congressional leaders, and said the House and Senate would have to govern on their own if Trump "can't participate constructively".

"Failure to do things like keeping the government open and passing a tax bill is the functional equivalent of playing Russian roulette with all the chambers loaded," Gregg said.

Others in the party divide blame between Trump and McConnell. Al Hoffman, a former finance chairman of the Republican National Committee who has been supportive of McConnell, said McConnell was culpable because he has failed to deliver legislative victories. "Ultimately, it's been Mitch's responsibility, and I don't think he's done much," Hoffman said.

But Hoffman predicted that McConnell would likely outlast the president.

"I think he's going to blow up, self-implode," Hoffman said of Trump. "I wouldn't be surprised if McConnell pulls back his support of Trump and tries to go it alone." An all-out clash between Trump and McConnell would play out between men whose strengths and weaknesses are very different. Trump is a political amateur, still unschooled in the ways of Washington, but he maintains a viselike grip on the affections of the Republican base. McConnell is a soft-spoken career politician, with virtuoso mastery of political fundraising and tactics, but he had no mass following to speak of.

McConnell, while baffled at Trump's penchant for internecine attacks, is a ruthless pragmatist and has given no overt indication that he plans to seek more drastic conflict. Despite his private battles with Trump, McConnell has sent reassuring signals with his public conduct: On Monday, he appeared in Louisville, Kentucky, with Steven Mnuchin, the Treasury secretary, for a discussion of tax policy.

McConnell's Senate colleagues, however, have grown bolder. The combination of the president's frontal attacks on Senate Republicans and his claim that there were "fine people" marching with white supremacists in Charlottesville has emboldened lawmakers to criticise Trump in withering terms.

Senator Bob Corker of Tennessee rebuked Trump last week for failing to "demonstrate the stability nor some of the competence" required of presidents. On Monday, Senator Susan Collins of Maine said in a television interview that she was uncertain Trump would be the Republican presidential nominee in 2020.

There are few recent precedents for the rift. The last time a president turned on a legislative leader of his own party was in 2002, when allies of George W. Bush helped force Trent Lott to step down as Senate minority leader after racially charged remarks at a birthday party for Senator Strom Thurmond.

For the moment, McConnell appears to be far more secure in his position, and perhaps immune to coercion from the White House. Republicans are unlikely to lose control of the Senate in 2018, and Trump has no allies in the Senate who have shown an appetite for combat with McConnell.

Still, some allies of Trump on the right - including Steve Bannon, who stepped down last week as Trump's chief strategist - welcome more direct conflict with McConnell and congressional Republicans.

Roger Stone Jr., a Republican strategist who has advised Trump for decades, said the president needed to "take a scalp" in order to force cooperation from Republican elites who have resisted his agenda. Stone urged Trump to make an example of one or more Republicans, like Flake, who have refused to give full support to his administration.

"The president should start bumping off incumbent Republican members of Congress in primaries," Stone said. "If he did that, Mitch McConnell and Paul Ryan would wet their pants and the rest of the Republicans would get in line." But McConnell's allies warn that the president should be wary of doing anything that could jeopardise the Senate Republican majority.

"The quickest way for him to get impeached is for Trump to knock off Jeff Flake and Dean Heller and be faced with a Democrat-led Senate," said Billy Piper, a lobbyist and former McConnell chief of staff.