Senators were expected to take up and pass the breakthrough bill later Thursday, and then send it to the House of Representatives -- which will barely have time to debate it before government funding expires at midnight.
The six-week spending bill, which also raises the federal debt ceiling, would not only avert a second shutdown in three weeks, but would break the cycle of government funding crises in time for what is set to be a bruising campaign for November's mid-term elections.
But a new wrinkle emerged as Republican Senator Rand Paul held up a speedy vote in opposition to the package's substantial increase in federal spending limits.
"All Senator Rand Paul is asking for is a 15-minute vote on his amendment to restore the budget caps," aide Sergio Gor said on Twitter. "He is ready to proceed at any time."
Moving legislation quickly through the Senate requires consent by all 100 members, and with barely eight hours before the deadline, Senate leadership was leaning on Paul to drop his insistence on an amendment.
"I think it will all work out, but it's up in the air," number two Senate Republican John Cornyn told reporters.
Even if Paul relents and the measure passes the Senate, its fate in the House is uncertain, raising tensions as Congress scrambles to avoid what would be a second government shutdown in three weeks.
Fiscal conservatives in the House may balk at adding billions of dollars to the national debt two months after passing a $1.5 trillion tax cut package.
With party unity fraying, House Speaker Paul Ryan appeared to shrug off concerns that several Republicans might oppose the deal.
"I think we're going to be fine," Ryan said in a radio interview about the upcoming vote.
The temporary spending bill incorporates the major budget deal struck Wednesday between Senate leaders on both sides of the political aisle.
That agreement includes a $300 billion increase to both military and non-military spending limits for this year and 2019, and raises the debt until March 1 next year.
The bill also provides a massive $90 billion in disaster relief following deadly 2017 storms in Puerto Rico, Florida and Texas, and funding to address the nationwide opioid abuse crisis.
"Not only will it end this series of... fiscal crises that have gridlocked this body, it will also deliver a large investment in our military and robust funding of middle-class programs," the Senate's top Democrat Chuck Schumer told colleagues.
'Real commitment' on immigration
The House Freedom Caucus of far-right Republicans has signalled possible roadblocks ahead by opposing the budget caps deal.
"We support funding our troops, but growing the size of government by 13 percent is not what the voters sent us here to do," the group said on Twitter Wednesday.
Liberal stalwarts were also in revolt because the deal does nothing to protect young undocumented immigrants from deportation.
Democrats have been seeking to link the federal funding debate to a permanent solution for hundreds of thousands of "Dreamer" immigrants who were brought to the country illegally as children.
Top House Democrat Nancy Pelosi -- who has vowed to oppose the temporary bill -- highlighted the concerns Wednesday with an extraordinary eight-hour address in which she demanded Ryan take action on immigration.
Dreamers were shielded from deportation under the Obama-era program called Deferred Action for Childhood Arrivals (DACA). But Trump ended the program last September, setting March 5 as a deadline for resolving the issue.
Facing tightening numbers for Thursday's vote, Ryan said he was prepared to address the immigration issue head on.
"I know that there is a real commitment to solving the DACA challenge in both political parties. That's a commitment that I share," Ryan told reporters.
"To anyone who doubts my intention to solve this problem and bring up a DACA and immigration reform bill: do not."
The White House's current proposal -- one that would put 1.8 million immigrants on a path to citizenship, but also boost border security, and dramatically curtail legal immigration -- has been panned by Democrats.
Several bipartisan efforts have stalled.
(This story has not been edited by NDTV staff and is auto-generated from a syndicated feed.)
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