US President Donald Trump has told Democrats that Obamacare will "die sooner" without money the White House has offered in exchange for an agreement to fund a wall on the Mexico border.
The White House also accused Democrats of "holding national security hostage" by refusing to allow $1.5 billion funding for the wall.
It came as Republicans and Democrats had until midnight on Friday to agree a spending bill and avoid a shutdown of the government.
Mr Trump's White House has offered to include $7 billion in Obamacare subsidies to help those on low incomes pay for health insurance if Democrats in return for the wall funding.
Mr Trump said on Twitter: "Obamacare is in serious trouble. The Democrats need big money to keep it going - otherwise it dies far sooner than anyone would have thought.
"The Democrats don't want money from budget going to border wall despite the fact that it will stop drugs and very bad MS-13 gang members."
He added: "Eventually, but at a later date so we can get started early, Mexico will be paying in some form, for the badly needed border wall."
During the election campaign Mr Trump said that Mexico would pay for the building of the nearly 2,000-mile wall.
There was uncertainty whether Mr Trump would sign a spending bill which did not include the funding for the wall.
Homeland Security secretary John Kelly told CNN: "He will do the right thing for sure but I suspect he will be insistent on the funding."
Democrats accused Mr Trump of making "poison pill" demands and said he should "back off".
ObamaCare is in serious trouble. The Dems need big money to keep it going - otherwise it dies far sooner than anyone would have thought.— Donald J. Trump (@realDonaldTrump) April 23, 2017
The Democrats don't want money from budget going to border wall despite the fact that it will stop drugs and very bad MS 13 gang members.— Donald J. Trump (@realDonaldTrump) April 23, 2017
Eventually, but at a later date so we can get started early, Mexico will be paying, in some form, for the badly needed border wall.— Donald J. Trump (@realDonaldTrump) April 23, 2017
Nancy Pelosi, Democrat leader in the House of Representatives, told NBC: "The wall is, in my view, immoral, expensive, unwise. Democrats do not support the wall. Republicans on the border states do not support the wall. The burden to keep it (the government) open is on the Republicans."
Senator Richard Durbin, a Democrat from Illinois, said: "To think that he would consider shutting down the government over this outlandish proposal of a border wall, that would be the height of irresponsibility."
But Reince Priebus, Mr Trump's chief of staff, said: "I'm pretty confident we're going to get something that's satisfactory to the president in regard to border security within the current negotiations.
"It'll be enough in the negotiation to move forward either with construction or the planning, to get going on the border wall and border security."