WASHINGTON (AFP) - It may not yet be a full-blown divorce, but the relationship between Donald Trump and the Republican Party is fast unravelling, threatening the President's prospects of steering legislation through Congress.

Trump took barely 18 months to conquer the party of Abraham Lincoln and Ronald Reagan, swatting aside more traditional challengers as he seized the Republican White House nomination in last year's US election.

But six months into his presidency, his disconnect with many in his party is clear, after the departure of two party stalwarts from the White House and the public shaming of his own attorney-general.

And after a recent humiliating defeat in the Senate on health reforms, party insiders are warning Trump can expect similar setbacks unless he learns to work constructively with Republicans on Capitol Hill.

"His presidency will only be successful if he has allies throughout the government, and that includes the legislative branch," said Alex Conant, a former senior aide to Senator Marco Rubio.

"If he spends his entire presidency at war with Congress, it will be a very unfulfilling four years," added Conant, who classed the current relationship between party and president as "awkward."

With chief of staff Reince Priebus and White House spokesman Sean Spicer - both party insiders - exiting within a week of each other, few senior Republicans remain among Trump's inner circle, besides Vice-President Mike Pence.

And while Pence - a former lawmaker in the House - acts as a bridge to Congress, it's far from clear how much the President is seeking his advice.

After the health debacle, instead of regrouping with Republican senators to find a common way forward, Trump rebuked them on Twitter, saying they would be "total quitters" if they abandoned his reforms.

OFFENCE TAKEN

Establishment Republicans have long winced at the probe into Russia's efforts to influence the 2016 presidential race and the swirling allegations into whether his campaign colluded with the Kremlin.

More recently, they have taken offence at the President's attacks on his Attorney-General Jeff Sessions, a former Republican senator who has significant support among congressional conservatives.

As pressure mounts on Trump, senior figures are becoming bolder in their criticism.

Senator Jeff Flake has urged Republicans to speak out if the president plays to his populist base in ways that damage the party's ability "to speak to a larger audience."

As a former Republican National Committee chairman, Priebus had impeccable connections and so his ouster last week has deprived the White House of a valuable link with Capitol Hill.

Spicer - who went a week earlier - was also a long-time Republican insider who owed his position as Trump's mouthpiece more to his party connections than his close relations with the President.

In Priebus' place, Trump brought in John Kelly, a retired Marine Corps general who has little experience in dealing with Republican lawmakers in either the House of Representatives or Senate.

Other key White House staff - such as chief strategist Steve Bannon, a former head of the fiercely right-wing Breitbart News media outlet - are also far removed from the Republican establishment.

The President's chief economic adviser, former Goldman Sachs president Gary Cohn, is a registered Democrat who has donated to both parties.

Trump's daughter and son-in-law Ivanka and Jared Kushner, both senior advisers, are self-styled New York progressives.

The little trust that existed in the first place between Trump and the Republican establishment may well have been shaken in the President's first six months in office.

But Richard Keil, a former Washington-based strategist who now works with the Hill + Knowlton consultancy, said ties were not necessarily beyond repair as long as the two sides could identify shared goals, such as tax reforms.

"If he can help them deliver what they want, that repairs relations pretty quickly," Keil said.