(Adds details regarding negotiations deadline)
By Richard Cowan and Ayesha Rascoe
WASHINGTON, April 24 (Reuters) - U.S. President Donald Trump pressed Democrats on Monday to include funds for his promised 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 either to score his first big legislative win or be mired in a Washington stalemate as he marks 100 days in office 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.
Congressional leaders will likely have to decide by late on Tuesday whether negotiations are progressing enough to try to pass a spending bill funding the government through September, Senator Roy Blunt, a member of the Republican leadership and Senate Appropriations Committee, told reporters on Monday.
If negotiations have slowed or stalled, Congress could pursue a short-term extension of existing spending levels to avoid parts of the federal government shutting down at 12:01 a.m. (0401 GMT) on Saturday, giving lawmakers more time to reach a deal. Leading Democrats have said they would support such a measure only if talks are progressing.
Short-term funding measures, known as continuing resolutions, have been used to avert government shutdowns in the past. But in 2013, conservative Republicans forced a 17-day shutdown in a failed attempt to repeal Obamacare.
If no spending measure is in place before Saturday, government funds will halt and hundreds of thousands of the country's several million federal employees will be temporarily laid off.
Those in jobs deemed essential such as law enforcement are expected to keep working in the hope 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 will grind to a halt.
Although the White House said on Monday that another vote to repeal and replace Obama’s signature healthcare law could not come for weeks, Trump is dangling the prospect of funding some elements of the law, which enabled millions more Americans to secure healthcare coverage, in exchange for Democrats' support in the spending talks.
The White House has offered to include $7 billion in Obamacare subsidies that allow low-income people to pay for healthcare insurance in exchange for Democratic backing of $1.5 billion in funding to begin construction of a barrier on the U.S.-Mexico border.
Trump has argued that the wall, a major theme of his 2016 presidential campaign, is needed to stem the flow of illegal immigrants and drugs into the United States. In a Twitter message on Monday, Trump wrote: “If... the wall is not built, which it will be, the drug situation will NEVER be fixed the way it should be!”
Trump's demand that Congress include funds for the construction of the wall remains a White House priority, White House spokesman Sean Spicer said on Monday.
"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.
Senate Democratic leader Chuck Schumer on Monday called the wall "a pointless waste of taxpayer money.” 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.
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 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.
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.
(Additional reporting by Julia Edwards Ainsley, Susan Heavey and Doina Chiacu; Writing by Paul Simao and Amanda Becker; Editing by Frances Kerry and Peter Cooney)