A major U.S. national-security bill is at risk of spectacular collapse. What happens next?

A far-reaching legislative effort risks exploding in spectacular fashion in a Wednesday afternoon vote in the U.S. Senate.

Fragments of unfinished business would be left strewn across the political landscape: Migration reform, weapons for an increasingly desperate Ukraine, and security aid for Taiwan and Israel.

It's all part of a sweeping national-security bill Republicans spent months negotiating, a bill with numerous Republican priorities, backed by the Republican-supporting Border Patrol union, and by Republican Wall Street Journal editorialists, and it could soon be killed — by Republicans.

"Why? A simple reason: Donald Trump," U.S. President Joe Biden said Tuesday. "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. American lawmakers are already looking past the near-certain failure of the bill for ways to salvage its broken pieces.

Here's how we got here.

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

Joe Biden calls on Americans to unite behind Israel, Ukraine

4 months ago

Duration 2:11

Joe Biden, in a rare primetime TV address from the Oval Office, calls on Americans to unite behind Israel and Ukraine — allies that depend on U.S. military aid.

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 does is ramp up border enforcement in numerous ways, albeit not as much as Republicans wanted.

It restricts which migrants can be released into the U.S. It makes it harder to seek asylum, speeds up the asylum process so applicants don't linger in the country, adds resources for detention and deportation, and quickens expulsions.

There's a border-shutdown provision: If the U.S. receives more than 5,000 migrants per day, the president is forced to halt migrant processing entirely.

For U.S. allies, there's $60 billion US for weapons to Ukraine and billions more to Israel for missile defence.

What did progressive Democrats get? Not much, in terms of their migration priorities. There's 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 hasn't stopped 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 will oppose the bill merely because Trump does; 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 several 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 doesn't force the completion of Trump's Mexico border wall. Also, in the above-mentioned border-shutdown provision, there's 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 provides up to $1.4 billion to the Federal Emergency Management Agency (FEMA) for use by NGOs, which help undocumented migrants.

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

In any case, it all appears 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.

But Republican leaders in the Senate have all but conceded defeat. In fact, they're now suggesting they'll vote against it because it's pointless.

And they're facing a rebellion just for trying to pass it: A handful of Senate Republicans, including Ted Cruz, held a news conference to trash their own leadership.

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

The Senate must vote on whether to advance the bill to the debate stage. The vote is expected Wednesday afternoon and needs 60 per cent to pass, but there's no sign of Republicans delivering the necessary votes.

Top Republicans appeared to have declared the bill dead, with McConnell saying the politics had changed and there was no real chance of it becoming law.

But then, he and several Republicans suggested 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.

When asked if he'd be willing to do that, Senate majority leader Chuck Schumer replied, cryptically, that he would have more to say if, and when, the scheduled vote failed. "Stay tuned."

Even then, there's no guarantee a Ukraine-Taiwan-Israel bill would get through the House of Representatives, as it would still require one of two uncertain developments.

The Republican speaker would have to allow a vote, and court a career-threatening backlash; or backbenchers would have to deploy a rarely-successful tactic called a discharge petition.

In the meantime, Schumer called it a gloomy moment for the U.S. Congress as Republicans ditched their principles to help Trump "and his friend … Vladimir Putin."

Troubling global effects

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

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."