As opposition appeared to swell in the House and Sen. Rand Paul, R-Ky., threw up last-minute roadblocks in the Senate, White House Office of Management and Budget spokesman John Czwartacki said that "agencies are now being urged to review and prepare for lapse" in spending after midnight.
Paul, making use of Senate rules that give individual senators enormous power to slow down proceedings that often require the consent of all, demanded a vote on his amendment that would demonstrate how the two-year budget deal breaks past pledges to rein in federal spending.
"I can't in all good honesty, in all good faith, just look the other way because my party is now complicit in the deficits," Paul said on the Senate floor as evening pushed into night, after objecting as Senate Majority Leader Mitch McConnell, R-Ky., tried to move to a vote.
Paul then launched into a lengthy floor speech deriding bipartisan complicity on deficit spending while the country goes "on and on and on finding new wars to fight that make no sense." Paul predicted a "day of reckoning," possibly in the form of the collapse of the stock market.
Senate leaders remained confident the spending deal would pass easily in the end. But absent an agreement among all senators on timing, final passage would be delayed until early Friday and the federal government could begin to shut down, at least briefly.
Even bigger problems appeared to be surfacing in the House, where liberals led by Minority Leader Nancy Pelosi, D-Calif., were incensed that the plight of young undocumented immigrants who face the threat of deportation was not addressed in the spending bill.
Pelosi planned to vote against the bill, and despite initially suggesting that she would not be urging fellow Democrats to follow her lead, she increasingly appeared to be doing exactly that.
At a closed-door evening meeting of House Democrats, Pelosi told lawmakers: "We have a moment. They don't have the votes. All of us should use our leverage. This is what we believe in," according to one House Democrat in the room, who demanded anonymity to disclose the private conversation.
Pelosi is under intense pressure from immigrant activists and liberals in her caucus to take a stand for the "dreamers," undocumented immigrants brought to the country as children who face losing work permits granted by former president Barack Obama but rescinded by President Donald Trump.
Supporters of these immigrants have watched in growing outrage as Democrats have failed repeatedly to achieve results for the cause. They want to see Democrats stand strong, even after a three-day partial government shutdown over the issue last month failed to achieve more than a commitment from McConnell to debate the issue on the Senate floor.
But many House Democrats are skittish over forcing another shutdown after the last one failed to yield much, especially with Senate Democrats largely on board for the spending deal. Minority Leader Charles Schumer, D-N.Y., negotiated the package with McConnell, with input from Pelosi and Speaker Paul Ryan, R-Wis.
"It's split in there, but not everybody has expressed their point of view," one lawmaker, Rep. Jackie Speier, D-Calif., said as she exited the House Democrats' meeting late Thursday.
House conservatives were also balking, objecting to the enormous increase in federal spending, most of which would be piled onto the deficit with minimal attempts to offset it.
Earlier Thursday, Ryan expressed confidence that the bill, which delivers a military funding boost sought by the GOP alongside increases in domestic spending favored by Democrats, would pass.
"There is widespread agreement in both parties that we have cut the military too much, that our service members are suffering as a result, and that we need to do better," he said. But the outlook was growing cloudier as the evening wore on. Lawmakers and aides said planning was underway to pass a very short-term spending extension to keep the government open past midnight, if necessary.
If the larger spending bill does pass, its impact would be wide-ranging- renewing several large health-care programs, suspending the national debt limit for a year and extending billions of dollars of expiring business tax breaks. The cost of those provisions exceeds $560 billion, though lawmakers included some revenue-raising offsets, such as increases in customs fees and a sell-off from the Strategic Petroleum Reserve.
In comparison, the 2009 fiscal stimulus bill passed at the bottom of a global recession under Obama was estimated to cost $787 billion over 10 years. Republicans were nearly unanimous in opposing that measure in their clamor for fiscal restraint in the face of growing deficits - demands largely drowned out now in the Trump era.
This spending bill, proposed amid an economic boom, could be the last major piece of legislation passed before November's midterm elections, barring a breakthrough on a thorny immigration debate.
In the House, Ryan, who wrote several deficit-cutting Republican budgets before becoming speaker, sought to tamp down fears that the bill could further explode the nation's fiscal imbalance by amping up spending without spelling out offsetting cuts or revenue-raisers.
Discretionary spending - the funding Congress doles out on a year-to-year basis for the Pentagon as well as for programs such as transportation, medical research and national parks - is not the main driver of the national debt, he argued, but rather "entitlement" programs such as Social Security, Medicare and Medicaid, which are left largely untouched by the pending deal.
"The military is not the reason we've got fiscal problems, it is entitlements," he said, adding, "You get entitlement reform, you can solve a lot of these problems."
But the massive spending bill, coming less than two months after Republicans pushed through a tax cut that stands to slash federal revenue by a trillion dollars or more over a decade, has given plenty of Republicans heartburn.
Ryan suggested in a radio interview Thursday that he would be able to deliver a majority of Republicans - about 120 votes - meaning about half of the 193 Democrats might be necessary to pass the deal. That could be a tough sell among House Democrats livid over the "dreamers" issue.
Pelosi delivered a record-breaking eight-hour speech on the House floor Wednesday centered on the immigration issue, demanding assurances that immigration legislation would be debated in the House before the fiscal deal was agreed to.
While Pelosi's speech was a potent gesture of support for the dreamers, she did not initially appear willing to whip her caucus against the budget deal. She was among the top leaders who negotiated the accord, and she has spoken positively about its domestic spending increases and other provisions.
That ambivalence was on display in a letter she sent to colleagues Thursday afternoon that set out "the reasons I am voting against this bill" while also noting that "Democrats successfully secured hundreds of billions of dollars in new investments." It did not explicitly urge Democrats to vote against it. Her position was causing confusion among fellow Democrats and appeared to be changing.
Ryan on Thursday delivered a new version of his previous pledges, saying "we are committed to getting this done" - but not without conditions. "We will bring a solution to the floor, one the president will sign," he said.
Democratic leaders have sharply rejected the outlines of an immigration bill put forth by the White House, leaving the prospects for a bill unsettled.
A number of House Democrats were unwilling to stand in the way of other party priorities to secure an immigration deal. Rep. Marcia Fudge, D-Ohio, predicted earlier Thursday that the budget agreement would pass "overwhelmingly" on a bipartisan vote.
"I can't go home to tell health centers that have already been handing out pink slips I didn't vote for this, and they gave you money for a permanent fix to your problem," she said. "I can't go home and say to union people: Look, they're going to try to take care of your pension problem, but I didn't vote for it. It just doesn't make any sense."
Under the deal, existing spending limits would be raised by a combined $296 billion through 2019. Those caps were put in place in 2011 after a fiscal showdown between Obama and GOP congressional leaders, who demanded spending austerity.
Bipartisan deals raised the caps in 2013 and 2015, and the new agreement is the first to be struck under unified Republican control of the White House and both chambers of Congress.
The agreement includes an additional $160 billion in uncapped funding for overseas military and State Department operations, continuing a costly line item that dates back to the immediate response to the 2001 terrorist attacks. And about $90 billion more would be spent on disaster aid for victims of recent hurricanes and wildfires. Tax provisions would add another $17 billion to the cost of the bill.
Some of the funding is reserved for programs favored by lawmakers of both parties: research conducted by the National Institutes of Health, for instance, as well as transportation and water infrastructure. Also included are extensions of tax breaks that could add billions of dollars more to the cost of the bill.
The bill also includes a provision suspending the federal debt limit until March 1 of next year - after November's midterm elections.
The Children's Health Insurance Program would be extended through 2028, and the federal fund for community health centers would see a two-year extension. The bill also abolishes the Independent Payment Advisory Board, a body established in the 2010 Affordable Care Act with the power to reduce the payments Medicare makes to health providers.
The legislation setting out the deal is expected to contain yet another deadline, March 23, giving congressional appropriators time to negotiate the fine details of funding agencies for the remainder of 2018.
Trump gave the accord a strong endorsement in a Wednesday tweet, saying it would give Defense Secretary Jim Mattis "what he needs to keep America Great" and calling on lawmakers of both parties to "support our troops and support this Bill!"
But conservative organizations such as the Heritage Foundation and the Club for Growth railed against the spending boost. Leaders of advocacy groups funded by brothers Charles and David Koch said in a statement that the deal was "a betrayal of American taxpayers and a display of the absolute unwillingness of members of Congress to adhere to any sort of responsible budgeting behavior."
Many Republican lawmakers did not see their vote in those terms. Rep. Charlie Dent, R-Pa., who chairs a House Appropriations subcommittee and pushed for months for a broad spending accord, said the deal would get lawmakers off a "treadmill" of short-term funding patches.
"Frankly, it will free up time for us to deal with other issues," he said. "It provides for stability, certainty, predictability, and that's not a small thing."
(Except for the headline, this story has not been edited by NDTV staff and is published from a syndicated feed.)