By Julia Edwards Ainsley

WASHINGTON (Reuters) - With a deadline looming this week to avert a U.S. government shutdown, Congress returns to work on Monday as President Donald Trump leans on Democrats to include funding for his promised border wall with Mexico in spending legislation.

The Republican president took to Twitter on Sunday to warn Democrats that the Affordable Care Act, popularly known as Obamacare, could soon lose essential funding without Democratic support for a congressional spending plan to keep the government running.

Should talks fail, the government would shut down on Saturday, Trump’s 100th day in office. Trump, whose national approval rating hovered around 43 percent in the latest Reuters/Ipsos polling, is seeking his first big legislative victory.

“Obamacare is in serious trouble. The Dems need big money to keep it going - otherwise it dies far sooner than anyone would have thought,” Trump said in a Twitter post.

The healthcare law was former Democratic President Barack Obama’s signature domestic policy achievement, which Republicans are trying to repeal and replace.

The White House says it has offered to include $7 billion in Obamacare subsidies that allow low-income people to pay for health insurance in exchange for Democratic backing for $1.5 billion in funding to start construction of the barrier on the U.S.-Mexico border.

Trump made the wall a major element of his presidential campaign, touting its ability to help curb the flow of illegal immigrants and drugs into the United States.

The federal government’s funding is set to expire at 12:01 a.m. on Saturday. A spending resolution would need 60 votes to clear the 100-member Senate, where Republicans hold 52 seats.

Asked if Trump would sign a spending bill that does not include money for the wall, White House budget director Mick Mulvaney told Fox News on Sunday: “We don’t know yet.”

Internal estimates from the Department of Homeland Security have placed the total cost of a border barrier at about $21.6 billion.

Trump has said Mexico will repay the United States for the wall if Congress funds it first. But he has not laid out his plan to compel the Mexicans to pay, which Mexico’s government has insisted it will not do.

‘FLY IN THE OINTMENT’

A Republican congressional aide said Democrats may agree to some aspects of the border wall, including new surveillance equipment and access roads, estimated to cost around $380 million.

“But Democrats want the narrative that they dealt him a loss on the wall,” the aide said, adding it would be difficult to bring any Democrats on board with new construction on the southwest border.

Democrats showed no sign of softening their opposition to wall funding on Sunday and sought to place responsibility for any shutdown squarely on Trump and Republicans who control the House of Representatives and the Senate.

Democratic Senate leader Chuck Schumer warned Trump to stay out of the way if he wanted lawmakers to reach a deal before the deadline.

Schumer told a news conference on Sunday that aid negotiations between Republicans and Democrats in the House and Senate were going well.

“The only fly in the ointment is that the president is being a little heavy handed, and mixing in and asking for things such as the wall,” Schumer said.

(Reporting by Julia Edwards Ainsley; Additional reporting by Doina Chiacu, Steve Holland and David Morgan; Editing by Peter Cooney)