The US leader was to address the media at the White House after he appeared to reverse his support for the $1.3 trillion spending bill, amid unfavorable television coverage.
Trump’s administration had categorically said he supported the deal brokered by the Republican controlled Congress and passed in a dead-of-night vote, claiming it as victory before the 71-year-old’s change of heart.
Many lawmakers have already left Washington for two weeks recess, so a renegotiation is unlikely.
“I am considering a VETO of the Omnibus Spending Bill,” Trump tweeted, suggesting the deal did not do enough to help 800,000 immigrants holding a de facto amnesty, which he rescinded.
Trump has repeatedly tried to blame the looming end of the “DACA” program on Democrats. The Obama-era plan protects migrants brought illegally to America as children from deportation.
The president also vented that the spending bill did not fully fund his signature campaign promise to build “the BORDER WALL, which” he said “is desperately needed for our National Defense.”
If the president does not sign the budget before midnight Friday, hundreds of thousands of civil servants will be put on forced leave, national parks from the Grand Canyon to Yellowstone will close and non-essential services will stop.
Trump’s threat came hours after a host on conservative channel Fox News pilloried the deal as a Washington “swamp budget.”
The tweet caps yet another week of high drama at a White House that seems to lurch from crisis to crisis.
This week the former reality TV star replaced his national security advisor, launched a new trade fight with China, and needled investigators probing Russia election meddling.
At the same time Trump faces an almost unprecedented number of scandals from a defamation lawsuit, to allegations of two extramarital affairs.
– ‘Funds his priorities’ –
The spending package provided $1.6 billion for border security and construction or repair of nearly 100 miles (160 kilometers) of border fencing, but that was far less than Trump had been seeking.
It also set aside the issue of the so-called “Dreamers,” who are in legal limbo following the Trump administration’s repeal of DACA.
The program expired on March 5, but the issue is being fought in the courts. Attempts at a legislative fix collapsed in a previous round of negotiations to avert a government shutdown.
In a Congress riven by partisan feuding, passage early Friday of the massive bill to fund the US government through September was considered a rare achievement.
The centerpiece was a big increase in US defense spending to $700 billion dollars, up $61 billion, and a 10 percent hike in domestic spending, which would rise to $591 billion.
“This bill is so important on many fronts, from school safety and troop funding, to opioids and veterans care,” press secretary Sarah Sanders said Thursday.
Trump’s budget director Mick Mulvaney went one step further: “Let’s cut right to the chase. Is the president going to sign the bill? The answer is yes. Why? Because it funds his priorities.”
Five times since October, lawmakers have had to pass stopgap funding legislation to keep the government’s lights on. Twice this year the government was allowed to slip into shutdown.
A third lapse would be deeply embarrassing for a Republican-controlled Congress facing midterm elections in November.
Democratic Senator Richard Blumenthal said Trump “needs to drop his wildly reckless veto threats” and sign the bill now.
“Americans deserve leadership from the White House, not more self-inflicted chaos.”
Some conservative Republicans welcomed the move, saying the process was flawed from the start.
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