President Trump on Tuesday night asked Congress to amend the nearly $900 billion stimulus bill passed just one day before, describing the legislation as “a disgrace” and suggesting he would not immediately sign off on aid for millions of Americans.

In a video posted to Twitter, Trump called on Congress to increase the “ridiculously low” $600 stimulus checks to $2,000 and outlined a list of provisions in the overall package of legislation that he described as “wasteful spending and much more.” He did not mention that the $600 stimulus check idea came from his treasury secretary, Steven Mnuchin.

“I am also asking Congress to immediately get rid of the wasteful and unnecessary items from this legislation, and to send me a suitable bill, or else the next administration will have to deliver a covid relief package, and maybe that administration will be me,” Trump said.

The video landed like a sonic boom in Washington. His own aides were stunned. Congressional aides were stunned. Stock market futures quickly slumped on the prospect that the economic aid could be in doubt.

And the implications for what happens next could be severe. If he refuses to sign the bill, the government will shut down on Dec. 29. The $900 billion in emergency economic aid will be frozen, and the race for the two Senate seats in Georgia could also be upended.

House Speaker Nancy Pelosi (D-Calif.), however, quickly responded to the Twitter post by saying congressional Democrats would move as soon as Thursday, when the House is scheduled to meet for a brief pro forma session, to advance the $2,000 stimulus checks.

“Republicans repeatedly refused to say what amount the President wanted for direct checks,” she posted on Twitter on Tuesday night after Trump’s message. “At last, the President has agreed to $2,000 — Democrats are ready to bring this to the Floor this week by unanimous consent. Let’s do it!”

Senate Minority Leader Charles E. Schumer (D-N.Y.) also tweeted that he supported the idea of larger stimulus checks, but he blamed Republicans for preventing them from being included in the bill.

“We spent months trying to secure $2000 checks but Republicans blocked it,” Schumer wrote. “Trump needs to sign the bill to help people and keep the government open and we’re glad to pass more aid Americans need. Maybe Trump can finally make himself useful and get Republicans not to block it again.”

Logistically, though, it could prove difficult for Democrats and Trump to amend the bill and approve $2,000 checks in the next few days, or even weeks.

If any Republican in the House opposed Pelosi’s effort on Thursday, it would not pass. Such a change would also require Senate Republicans to pass the measure unanimously, something that is unlikely to happen.

Several White House aides spoke on the condition of anonymity to describe the chaotic and secretive process that unfolded Monday and Tuesday, when many of them were kept in the dark about Trump’s motives and the video. Trump’s chief of staff, Mark Meadows, kept the video very closely held on Tuesday, several of them said, with aides involved in the negotiations learning of it only an hour before it was posted. Even Trump’s legislative affairs office, which is responsible for dealing with Congress every day, was caught unaware, they said.

The $600 direct payments added roughly $167 billion to the $900 billion package, according to Ernie Tedeschi, an economist and former Treasury Department official in the Obama administration. Tedeschi estimated that increasing the payments to $2,000 per adult would grow the cost of the bill by $370 billion.

The 5,593-page package was introduced Monday afternoon and passed the House and Senate late with broad bipartisan support, clearing the Senate by a 92-to-6 margin. Republicans had insisted on keeping the economic relief portion at less than $1 trillion, and larger checks would have pushed the final tally higher.

Trump’s aides had made positive comments about the bill lawmakers passed, but Trump had largely stayed out of negotiations. Last week, he had complained to some aides that the $600 stimulus checks were too low and that he wanted them raised to $1,200 or $2,000, but aides had convinced him not to intervene, saying it could scuttle the whole package.

Some aides were stunned that Trump weighed in the way he did after his economic team had publicly praised the bill.

But administration officials had negotiated in the final days without explicitly securing Trump’s approval, aides said. He had largely been distracted with overturning the results of the presidential election.

Trump had long wanted to do more than $600 in checks and kept asking aides why they couldn’t agree to a bigger number, an official said.

He released the video Tuesday after a number of his aides, including Meadows, were already out of town.

“So dumb,” one administration official said. “So, so dumb.”

As the coronavirus pandemic began to move rapidly through the United States in March, Congress passed a $2.2 trillion relief bill to limit the economic impact. That law included the first round of one-time stimulus payments, pegged at $1,200.

Many of that law’s other measures expired over the course of the year, and the recent spike in cases — and the end of the election campaign — sparked a bipartisan coalition to seek a new bill. The measure that passed Monday night promised $900 billion in new assistance, including the $600 stimulus checks, enhanced unemployment aid for 11 weeks, small-business assistance and a range of other measures.

Trump’s top economic advisers had not signaled that he was unhappy with the bill. In fact, they had suggested they approved of the way the package came together.

“I am pleased that Congress has passed on an overwhelming bipartisan basis additional critical economic relief for American workers, families and businesses,” Mnuchin tweeted seven hours before Trump’s video was posted.

Mnuchin gushed about the stimulus bill Monday, and he serves as Trump’s key negotiator with Congress on spending and economic matters.

“Mnuchin (your Treasury Sec) represented YOU during extended negotiations on this bill package. We are Republic not a monarchy,” retiring Rep. Paul Mitchell of Michigan wrote on Twitter Tuesday night in a message aimed at Trump. Mitchell had served as a Republican lawmaker before announcing that he was disassociating from the party in part because of Trump’s antics since the election.

Aides told reporters all day that Trump would be signing the bill, but they later learned that he taped the video at least five hours before it was released, officials said.

Not all aides were supportive of the measure, though some kept their criticisms closely held.

Aides who dislike the bill used the fact that some of its unrelated spending provisions included foreign aid as a way to turn Trump against the measure, knowing that American money going to other countries raises the president’s ire. There was also a backlash to the bill among conservatives on Twitter, something Trump tends to monitor carefully.

Rep. Adam Kinzinger (R-Ill.) tweeted Tuesday night that Trump was “conflating” the government spending portion of the bill with the economic relief portion.

“HIS TEAM negotiatiated [sic] this, and blessed combining the two! But since twitter erupted, he erupted,” Kinzinger wrote.

Virtually all of the complaints Trump made in the four-minute video — including foreign aid agreements, aid to the Kennedy Center, fish management language and more — are not part of the $900 billion covid relief agreement but rather included in other, separately negotiated parts of the legislation, including a $1.4 trillion omnibus appropriations bill and a measure authorizing $9.9 billion in water projects. These bills and many others were packaged together.

Two congressional aides who had been involved in the negotiations said they were unaware of any problems the White House had with the bill. On Sunday, Ben Williamson, a spokesman for Meadows, said publicly ahead of the bill’s release that Trump supported the legislation and would sign it.

Trump’s refusal to publicly embrace the bill as it moved through Congress made some aides nervous, raising the prospect that he would once again cannonball into the pool at the end with a public declaration.

“This is what happens with a president who places more trust in conservative fever swamp Twitter than his own Treasury Secretary. His administration helped negotiate this bill, and he just pulled down the pants of every Republican who voted for it,” said Brendan Buck, a former top aide to onetime House Speaker Paul D. Ryan (R-Wis.). Ryan and Buck often struggled with the president changing his mind.

Trump has also not shared the fiscal discipline that some of his aides, such as Office of Management and Budget Director Russ Vought, have and has told advisers that doing only $600 would not be “popular,” in the words of one senior administration official.

The House and Senate passed the bill with such large margins that they could probably override a veto if Trump tried to block the measure. But that process could take weeks. Two aides said Trump might still sign the bill, noting he did not explicitly say he would veto it. Whether he goes to Mar-a-Lago on Wednesday remains unclear, several advisers said, even though it is on his schedule.

Some of Trump’s closest allies on Capitol Hill even tried to defend the economic relief bill Tuesday night, raising questions about who had convinced the president to turn against it.

“The #COVID19 package, while imperfect, will save jobs and lives,” Sen. Lindsey O. Graham (R-S.C.) tweeted. “The sooner the bill becomes law — the better.”

Some White House officials were scrambling late Tuesday to discern who got in Trump’s ear and helped make the video, along with giving him erroneous information about the bill.

- Jeff Stein contributed to this report.