A major U.S. national-security bill suffers spectacular collapse. What happens next?

American lawmakers are struggling to salvage broken pieces from a major national-security bill that crash-landed Wednesday in a spectacular legislative failure.

Fragments of unfinished business are now strewn across multiple continents: Migration reform, weapons for an increasingly desperate Ukraine, and security aid for Taiwan and Israel.

The failed vote already has congressional leaders turning to a plan B: Immediately holding a separate vote on funding for Ukraine, Taiwan and Israel in the hopes that it will fare better.

This is after a months-long effort that's become a poster child for the chronic inability of the U.S. Congress to find consensus on issues related to immigration.

It involves a bill that Republicans negotiated, a bill that included numerous Republican priorities. It's backed by major Republican-supporting institutions, the Border Patrol union and editorial page of the Wall Street Journal.

And now it's been killed — by Republicans.

"Why? A simple reason: Donald Trump," U.S. President Joe Biden said on the eve of the vote, anticipating its demise. "Because Donald Trump thinks it's bad for him politically."

Trump and his allies have been pushing elected Republicans to block the legislation, arguing that it helps Biden in an election year.

Here's how we got here.

WATCH | Biden's push for national-security bill: 

Anatomy of a 4-month saga

Last fall, Ukraine started running low on U.S.-supplied weapons. The Biden administration urged Congress to renew funding for a program with two goals: Send old U.S. weapons to Ukraine, and buy new ones for the U.S.

Republicans grew increasingly skeptical. Several asked the question: Why spend billions more protecting Ukraine's border, and not America's?

So, Biden suggested a compromise — put everything in one bill.

In a prime-time address after the Oct. 7 attacks on Israel, the president proposed a broad national-security law that would tighten American borders while delivering military assistance to Ukraine, Israel and Taiwan.

Negotiators spent four months working on it. They met nights, weekends and through the Christmas holidays, said a furious lead Democratic negotiator, Sen. Chris Murphy.

He fumed Tuesday that, even in Washington, where it's easy to lose one's capacity for outrage, "What's happened here the last four months is outrageous."

Murphy quoted one colleague who called this his most dispiriting week as a lawmaker.

Lead Republican negotiator James Lankford acknowledged that it felt like his colleagues had thrown him under the bus: "And backed up."

What the bill actually did is ramp up border enforcement in numerous ways, albeit not as much as Republicans wanted.

It would have restricted which migrants can be released into the U.S. It would have made it harder to seek asylum, speed up the asylum process so applicants don't linger in the country, add resources for detention and deportation, and quicken expulsions.

There was a border-shutdown provision: If the U.S. received more than 5,000 migrants per day, the president would have been forced to halt migrant processing entirely.

For U.S. allies, there was $60 billion US for weapons to Ukraine and billions more to Israel for missile defence. That's the part lawmakers are now trying to revive.

What did progressive Democrats get? Not much, in terms of their migration priorities. There was no reference to legal status for undocumented people already in the U.S. The closest the bill came to including a traditional liberal demand would be a provision for 50,000 new legal immigration spots per year over five years.

Trump lobbies against bill: 'Don't be STUPID!!!'

That didn't stop Trump from lobbying against the bill.

"This Bill is a great gift to the Democrats, and a Death Wish for The Republican Party," Trump posted on social media.

"Don't be STUPID!!!"

On Fox News, morning anchor Steve Doocy said some Republicans opposed the bill merely because Trump did; because they want chaos at the border to be an election issue.

"The Republicans want this through November so they can say, 'Joe Biden broke it,' " Doocy said.

But the loudest, most powerful voices on Fox News, prime-time hosts like Sean Hannity and Laura Ingraham, those most closely aligned with Trump, have been vigorously strafing the bill.

"An unmitigated disaster," Hannity said this week. "Every Republican should strongly be opposed to this."

What Republicans don't like

There are elements Republicans dislike. It's not nearly as restrictive as a separate border bill, which Republicans passed months ago in the House but that doesn't stand a chance of passing the Senate.

For starters, this bill didn't force the completion of Trump's Mexico border wall. Also, in the above-mentioned border-shutdown provision, there was a loophole allowing the president to suspend the measure for up to 45 days if he deems it in the national interest.

The bill also provided up to $1.4 billion to the Federal Emergency Management Agency (FEMA) for use by NGOs, which help undocumented migrants.

While it tightened the ways migrants could be temporarily released into the U.S., it did not set a hard maximum cap on use of so-called parole.

In any case, it's doomed.

Republican leadership facing rebellion

It was always a longshot that the bill would be passed in the Republican-led House. But some advocates hoped it would get through the Senate, then either through negotiation or some parliamentary procedural manoeuvre, someone might force a vote on the House floor.

Republican leaders in the Senate conceded defeat. Almost all Republicans opposed the bill Wednesday in a 49-50 vote, far short of the 60 required to advance it; some progressive Democrats also opposed it.

GOP leaders are facing a rebellion just for working on the bill. A handful of Senate Republicans, including Ted Cruz, held a news conference to trash their own leadership.

WATCH | A new era for the Republican Party: 

Cruz suggested it was time for Mitch McConnell to resign as leader, blaming him for forcing the party into an embarrassing predicament.

Now Republicans and some Democrats are working on a next step: Holding a separate vote on the foreign-assistance portion, on Ukraine, Israel and Taiwan.

Next step: Break up the bill?

"We still, in my view, ought to tackle the rest of it. Because it's important. Not that the border isn't important — but we can't get an outcome," McConnell said.

There's no guarantee a plan B will pass, and the effort is already off to a bumpy start. The pared-down bill was subjected to an hours-long vote delay amid a dispute on the Senate floor about whether it could be amended eventually. The vote was paused overnight and rescheduled for Thursday.

Then there's the even taller hurdle: the House of Representatives. Simply getting a vote in that chamber on a Ukraine-Taiwan-Israel bill would require one of two uncertain developments.

Either the Republican House speaker would have to allow it, risking a career-threatening backlash, or, after 30 days, backbenchers could attempt a rarely-successful parliamentary tactic called a discharge petition.

Troubling global effects

A former CIA analyst who now sits in Congress said she came back from a troubling Pentagon briefing this week about some of the potential global effects of the stalled legislation.

Elissa Slotkin described Ukraine being pummeled without adequate defences; its energy and grain-export infrastructure damaged; higher global food prices as a result; a wave of refugees fleeing advancing Russian troops; and historic humiliation for the U.S.

"Historians may remember this as the moment when America gave up on defending democracy," the Michigan Democrat posted on X, formerly Twitter.

"When our political polarization got so bad that we abandoned the principles our grandparents fought for."