Washington: Discounting an extraordinary, Brexit-like moment in American politics, senior Republican Party figures seem resigned to losing the presidency and both houses of Congress come November – at least while Donald Trump remains their presidential candidate.

In the latest sign that the wheels are falling off Trump's campaign, his running mate and Indiana governor Mike Pence is explicitly splitting with Trump on key issues: on Tuesday, Pence endorsed GOP Speaker Paul Ryan's re-election bid, which Trump refuses to do; and days earlier Pence refused to join Trump's war on the family of a Muslim soldier killed in action in Iraq, opting instead to speak well of him.

Trump's appalling assault on the Khan family appears to have been a game-changer for Republicans, many of whom have been muted in their criticism and few of whom have been brave enough to revoke their endorsement.

But on Wednesday, Congressman Adam Kinzinger, of Illinois, did just that, declaring: "Donald Trump is beginning to cross a lot of red lines of the unforgivable in politics. I'm not going to support Hillary, but in America we have the right to skip somebody. That's what it's looking like for me today. I don't see how I get to Donald Trump anymore."

And a day earlier, Richard Hanna, a retiring congressman of New York, endorsed Clinton, declaring Trump "unfit to serve".

Absurdly, just hours before Pence publicly endorsed Ryan, Trump tweeted: "There is great unity in my campaign – perhaps greater than ever before."

That the party is in turmoil comes as no surprise, given that Trump lurches from one crisis to the next, obsessing about himself and missing openings that a more practiced candidate would have used to launch attacks on his Democratic rival, Hillary Clinton.

But that senior figures are saying so, some publicly, and openly canvassing their options is a measure of desperation after months of them reassuring would-be campaign donors and voters that Trump would pivot to a more conventional campaign persona.

Warning that Trump risks throwing away the election, one of his most loyal supporters, former House speaker Newt Gingrich, told The Washington Post: "[He] is helping [Clinton] to win by proving he is more unacceptable than she is."

Giving Trump just weeks to reposition or lose, Gingrich said: "Anybody who is horrified by Hillary should hope that Trump will take a deep breath and learn some new skills – he can't win the presidency operating the way he is now. She can't be bad enough to elect him if he's determined to make this many mistakes."

People close to Republican National Committee (RNC) chairman Reince Priebus are telling reporters that he is stressed and "very frustrated" that his sustained counselling of Trump has had no effect – and left him bereft of excuses for the candidate's wild and seemingly senseless conduct.

"A new level of panic hit the street," Scott Reed, chief strategist for the US Chamber of Commerce, told the Post. "It's time for a serious reset," he said, urging Trump to "stop doing silly interviews nine times a day that get you off message."

Party rules don't allow for the most extreme option – dumping Trump as the candidate.

But preparatory legal work has begun on how to deal with him deciding to quit – either of his own volition or under pressure, according to ABC News. In either case, it would fall to the 168-member RNC to pick a new candidate.

Instead there is talk of an "intervention", possible by a group of high-profile Republicans and by Trump's children, in the hope of settling Trump into a more conventional, offensive campaign against Clinton.

In the days after Trump clinched the nomination, senior Republicans were unnerved when, instead of marketing himself as a serious presidential possibility, Trump turned gutter fighter, launching relentless attacks on the judge hearing claims on Trump's dodgy university venture which were seen as racist – even by Republicans.

They started breathing again by the end of the Republican convention in Cleveland, wanting to believe that Trump's egocentric convention performance had hints of what they expect from a candidate.

But since the end of the Democratic jamboree which confirmed Clinton's candidacy almost a week ago, Trump has been merciless in attacking the parents of Captain Humayun Khan, who died fighting for his country in Iraq – instead of turning to exploiting and exposing Clinton's many weaknesses.

"I'm pulling for [Trump]. But he's not driving on the pavement – he's in the ditch," Henry Barbour, an RNC member and veteran strategist in Mississippi, told the Post. "I'm frustrated. There's time to fix it, but there's [only] one person who can fix it. It's up to him. Republican voters have empowered him to be our nominee and he's the one who's got to perform.

"The last several days have made this election a referendum on Donald Trump. We want it to be a referendum on Hillary Clinton."

Barbour's anxiety is replicated across the country, with Senate and House candidates and their strategists despairing over the party's chances of holding its congressional majorities.

Trump's missed opportunities to go after Clinton, they say, include data released on Friday showing slower-than-anticipated economic growth and a TV interview in which Clinton argued, contrary to the facts, that the FBI had agreed with her account of her controversial private email server, both of which were invitations to go on the offensive – and both of which were ignored by Trump.

In a rare nugget of good news for Trump, his campaign announced on Wednesday that a flood of small donations had virtually caught him up to the prodigious fundraising effort of the Clinton campaign – in July, Trump and the RNC had raised a combined $US82 million.

But it was accompanied by more of the usual gloomier reports – a poll in Florida showed his support among Hispanics to be just 12.9 per cent, compared with about 40 per cent for the Republican candidate in the last two elections. And nationally, Clinton had a 10-point lead on him in a new Fox News poll.

States that ordinarily would be a shoo-in for the GOP are becoming less certain. Polling in Arizona shows Clinton in the lead and in Utah, local GOP officials are sweating it – both states have significant communities of Mormons who have registered their disgust with Trump.

Worries about Trump hitting the chances for down-the-ticket GOP candidates have led party officials to a desperate hope that voters will split the ticket – vote against Trump, but tick the box for the rest of the GOP slate. But experts are sceptical.

"In 2008 and 2010, voters did not draw distinctions," Fergus Cullen, a former chairman of the New Hampshire Republican Party told The New York Times. "It was not like Passover, where the door was marked, 'This one should be spared.' No, the Angel of Death came in and said 'Let's kill them all'."

Meanwhile, Trump goes on his merry way – he didn't know what Brexit was, but maybe he feels it.

Having already insulted the disabled, war heroes and the families of the fallen, Muslims and Hispanics, he turned his attention this week to babies – and by association, to women [again].

When a baby in the audience started to cry during his speech to a rally in Virginia on Tuesday, Trump behaved as a model candidate: "Don't worry about that baby. I love babies ... I hear that baby crying and I like it. I like it. What a baby, what a beautiful baby.

"Don't worry, don't worry ... It's young and beautiful and healthy and that's what we want, OK."

But just minutes later, as the baby continued bawling, Trump went nasty: "Actually I was only kidding. You can get the baby out of here."

And speaking of the unfortunate mother, Trump blathered on in his typical un-empathetic way: "I think she believed me that I love having a baby crying while I'm speaking. That's OK. People don't understand."