By Ayesha Rascoe and Richard Cowan
WASHINGTON (Reuters) - U.S. President Donald Trump tried to press Democrats on Monday to include funds for his controversial border wall with Mexico in spending legislation as lawmakers worked to avoid a looming shutdown of the federal government.
The battle offers the Republican president, whose approval ratings have slid since he took office, a chance to score his first big legislative win or to be mired in a Washington stalemate as he marks 100 days in the job on Saturday.
Republicans control both chambers of Congress, but a White House-backed bill to gut former President Barack Obama's Affordable Care Act, popularly known as Obamacare, failed to gather full party support and imploded last month.
If no deal is agreed on spending, parts of the federal government will shut down at 12:01 a.m. (0401 GMT) on Saturday.
In the past, such shutdowns have generally been averted through passage of funding measures known as continuing resolutions. But in 2013, conservative Republicans forced a 17-day shutdown in a failed attempt to repeal Obamacare.
When funds abruptly halt, hundreds of thousands of the country's several million federal employees are temporarily laid off, while those in jobs deemed essential such as law enforcement are expected to keep working in the hope that they will receive back-pay. Non-essential sectors such as national parks are liable to be closed and programs such as federally funded medical research grind to a halt.
Trump's demand that Congress include funds for the construction of the wall, a key theme of his 2016 presidential campaign, remains a White House priority, White House spokesman Sean Spicer said.
"The president has made very clear that he's got two priorities in this continuing resolution: No. 1, the increase in funding for the military and No. 2, for our homeland security and the wall," Spicer told reporters.
The White House is confident in the direction of the talks and an announcement is expected soon, Spicer said, although he declined to say specifically whether Trump would sign a bill that did not contain money for border security and the wall.
A funding bill will need 60 votes to clear the 100-member Senate, where Republicans hold 52 seats, meaning at least some Democrats will have to get behind it.
Trump, who argues that a wall is necessary to stem the flow of illegal immigrants and drugs into the United States, said in a Twitter message earlier on Monday that "If ... the wall is not built, which it will be, the drug situation will NEVER be fixed the way it should be!"
Senate Democratic leader Chuck Schumer, calling the wall "a pointless waste of taxpayer money," said on Monday that illegal drugs would still flow into the United States via tunnels and vehicle traffic.
He repeated an assertion made last week that bipartisan negotiations in Congress were going well until the White House began demanding money for the wall as a condition for accepting a funding bill.
Trump has said Mexico will repay the United States for the wall if Congress funds it first. But the Mexican government is adamant it will not provide any financing and Trump has not laid out a plan to compel Mexico to pay. Department of Homeland Security internal estimates have placed the total cost of a border barrier at about $21.6 billion.
Aside from inflaming relations with a major trading partner, the planned wall has angered Democrats. They showed no sign of softening their opposition on Monday and sought to place responsibility for any shutdown squarely on Trump and congressional Republicans.
Republican aides in Congress provided no timetable for the unveiling of a bill to fund the government from April 28 to Sept. 30.
One senior Republican congressional aide said that if not enough progress is made by Thursday, Congress would likely have to try to push forward a stop-gap spending bill to keep the government operating. Leading Democrats have said they would support such a measure only if there was progress in the talks.
Failure to approve a government funding bill could throw new doubts over Republicans' ability to fashion a budget blueprint for the fiscal year that starts on Oct. 1 or to succeed in a major effort to cut corporate and individual taxes that Trump has touted.
A Republican congressional aide said over the weekend that 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.
Trump is dangling the prospect of funding some elements of Obamacare in exchange for Democrats' support in the spending talks, while goading opponents with predictions of the imminent demise of Obama's signature domestic achievement, which enabled millions more Americans to secure healthcare coverage.
"ObamaCare is in a death spiral!" he said in another tweet on Monday.
(Reporting by Julia Edwards Ainsley and Ayesha Rascoe; Additional reporting by Susan Heavey, Doina Chiacu and Richard Cowan; Writing by Paul Simao; Editing by Frances Kerry)