WASHINGTON (Reuters) - Republicans in the U.S. House of Representatives on Thursday added $5 billion to a government spending bill to help President Donald Trump make good on a pledge to build a border wall, a move that made a partial federal government shutdown more likely this weekend.

The Senate is highly unlikely to pass the legislation, which funds agencies responsible for federal law enforcement activities, airport security screenings, space exploration and farm programs, by a midnight deadline on Friday.

Trump had said he would not sign a Senate-passed bill to keep the government running through Feb. 8 because it lacked funds for the wall, a signature promise of his 2016 run for office, so Republicans in the House of Representatives scrambled to add money to appease the president.

Trump demanded $5 billion to put toward a wall on the U.S. border with Mexico that he argues is needed to keep out illegal immigrants and drugs, a down payment on a massive project which Democrats have rejected as ineffective and wasteful.

“The bill that’s on the floor of the House, everyone knows will not pass the Senate,” Democratic Leader Chuck Schumer told reporters ahead of the vote.

The funding uncertainty weighed on markets on Thursday but it was later dwarfed by another bombshell from the Trump administration: the resignation of Defense Secretary Jim Mattis.

Trump, who accused Democrats of playing politics with the border wall, has said he sees it as a winning issue for his 2020 re-election campaign. Last week in a White House meeting with Democratic congressional leaders, he said he would be “proud to shut down the government for border security.”

“I’ve made my position very clear. Any measure that funds the government must include border security,” Trump said at a White House event.

The House-passed bill also includes $7.8 billion in aid for states hit by recent natural disasters.

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The Senate had approved a bill late Wednesday that extended existing funds for agencies for seven weeks, punting the funding issue into the next Congress.

But hard-right conservative pundits and lawmakers urged Trump to push for border wall funding now, even if it leads to a shutdown, arguing that it would be impossible to get once Democrats take control of the House on Jan. 3.

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“It’s really about a president that is not willing to fold without a fight,” Republican Representative Mark Meadows said in an interview on Fox News.

The Senate will look at the House-passed bill on Friday afternoon but would need support from Democrats to vote quickly on it. Most senators had already left Washington for the Christmas break.

Trump also had planned to leave for a vacation at his private resort in Florida, but the White House said he would not go in the event of a shutdown.

During his 2016 campaign, Trump repeatedly said Mexico would pay for the border wall but never specified how. Last week he said that a new trade deal his administration had negotiated with Mexico and Canada would generate enough savings to be equivalent to Mexico financing the wall.

The issue of money for the wall also led to a partial government shutdown during a weekend in January, and a second funding gap in February lasted only a few hours. Impact on government services was negligible in both cases.

In a 2013 shutdown, conservative Republicans made an unsuccessful effort to use funding bills to repeal President Barack Obama’s healthcare law.