A small, but significant slice of the American electorate may hold the key to US President Donald Trump’s political future.

Call them the Trump reluctant, a group that represents roughly 2 in 10 voters who only “somewhat” approve of the Republican president’s job performance.

That’s according to VoteCast, The Associated Press’ nationwide survey from the recent midterm electorate. The survey reveals warning signs for Trump’s Republicans among the small voting bloc with big political influence.

Compared with the 27 per cent of voters who describe themselves as strong Trump supporters, the “somewhat” Trump voters are much more likely to disapprove of Trump on issues such as immigration and health care, and much more likely to question his trustworthiness and temperament.

They’re also more likely to have voted for Democrats in 2018.

It comes as Mr Trump on Sunday announced he would replace Defence Secretary Jim Mattis with his deputy Patrick Shanahan, days after the outgoing Pentagon chief quit while citing key policy differences with the US president.

“I am pleased to announce that our very talented Deputy Secretary of Defence, Patrick Shanahan, will assume the title of Acting Secretary of Defence starting January 1, 2019,” the Republican leader tweeted, accelerating Mr Mattis’s planned departure by two months.

“Patrick has a long list of accomplishments while serving as Deputy, & previously Boeing. He will be great!”

Meanwhile, French President Emmanuel Macron has criticised Mr Trump’s decision to withdraw American forces from Syria, saying “an ally must be reliable”.

Speaking in the Chad capital N’Djamena, Mr Macron said “I deeply regret the decision” by Mr Trump to pullout US troops.

Mr Trump last week ordered a complete troop pullout from Syria, asserting that the Islamic State group had been defeated, and a significant withdrawal from Afghanistan.

“To be an ally is to fight shoulder to shoulder,” Mr Macron said, adding that France was doing just that in Chad in the fight against jihadist groups.

“An ally must be reliable, to co-ordinate with its other allies,” he said. Mr Macron also paid tribute to Defence Secretary James Mattis, who said he was resigning on Thursday after Mr Trump’s Syria announcement.

“I want here to pay tribute to General Mattis … for a year we have seen how he was a reliable partner,” Mr Macron said at a press conference with his Chadian counterpart Idriss Deby.

Mr Mattis, 68, was one of Mr Trump’s first cabinet picks and has spent nearly two years at the Pentagon.

In his resignation letter Mr Mattis spelled out to the world what seemed obvious to many observers: Mr Trump’s world view was irreconcilable with his own.

“My views on treating allies with respect and also being clear-eyed about both malign actors and strategic competitors are strongly held and informed by over four decades’ immersion in these issues,” Mr Mattis wrote to Mr Trump, who has sought closer ties with Russia and heaped contempt on NATO and other alliances.

“Because you have the right to have a Secretary of Defence whose views are better aligned with yours on these and other subjects, I believe it is right for me to step down from my position.”

Brett McGurk, the US special envoy to the anti-Islamic State group coalition, also announced he would bring his departure forward from February due to the Syria policy change.

It comes as the US government is expected to remain partially closed past Christmas Day in a protracted stand-off over Mr Trump’s demand for money to build a border wall with Mexico.

On the second day of the federal closure, Mr Trump tweeted on Sunday that what the country needs is “a good old fashioned WALL that works”, as opposed to aerial drones and other measures that “are wonderful and lots of fun” but not the right answer to address the problem of “drugs, gangs, human trafficking, criminal elements and much else from coming into” the US.

With Mr Trump’s insistence on $US5 billion ($A7b) for the wall and negotiations with Democrats in congress far from a breakthrough, even a temporary measure to keep the government running while talks continued seems out of reach until the Senate returns for a full session on Thursday.

White House chief of staff Mick Mulvaney said on Sunday the partial federal government shutdown could extend into January.

“It’s very possible the shutdown will go beyond the 28th and into the new congress,” Mr Mulvaney said on Fox News.

“I don’t think things are going to move very quickly here the next couple of days.”

From coast to coast, the first day of the shutdown played out in uneven ways. The Statue of Liberty was still open for tours, thanks to money from New York State, and the US Post Office was still delivering mail, as an independent agency.

Yet the disruption has affected many government operations and the routines of 800,000 federal employees.

No one knew how long the closures would last. Unlike other shutdowns, this one seemed to lack urgency, coming during the long holiday weekend after Mr Trump had already declared Monday, Christmas Eve, a federal holiday.

Rather than work around the clock to try to end the shutdown, as they had done in the past, the leaders of the House and the Senate effectively closed up shop. “I am in the White House, working hard,” tweeted the president on Saturday. He had cancelled his Florida holiday getaway to his club Mar-a-Lago because of the shutdown.