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.

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."