Former President Trump's dominance in the GOP primary has scrambled the political incentives underlying one of President Biden's top vulnerabilities: the border.

Why it matters: The logistical and humanitarian crisis along the U.S.-Mexico border is triggering constitutional challenges and a massive political headache in Congress — with election season only pouring more fuel on the fire.

• In Washington, Republican senators are working furiously to salvage a bipartisan border security package that Trump opposes because it could undermine one of his most potent campaign themes.
• In Texas, Republican Gov. Greg Abbott is openly defying a U.S. Supreme Court decision and arguing that Biden has failed in his constitutional duty to defend states from a border "invasion."

Driving the news: The Senate devolved into chaos after GOP Leader Mitch McConnell (R-Ky.) reportedly cast doubt on the pathway for a border deal in a closed-door meeting Wednesday.

• "We don't want to do anything to undermine" Trump, Punchbowl News quoted McConnell as saying.
• Republican demands to pass stricter border policies have held hostage tens of billions of dollars in military aid for Ukraine and Israel.

Zoom in: Several GOP senators who have supported the monthslong negotiations arrived to the Capitol on Thursday angry and confused.

• "The fact that [Trump] would communicate to Republican senators and congresspeople that he doesn't want us to solve the border problem because he wants to blame Biden for it is really appalling," Sen. Mitt Romney (R-Utah) told CNN.
• "Don't be a coward. Don't pretend that the policy isn't strong. If you want to admit you're just afraid to tell President Trump the truth, that's fine," Sen. Thom Tillis (R-N.C.) said of fellow Republicans.

Yes, but: Some senators said the coverage of McConnell's comments was overblown and misleading, and the GOP leader clarified his support for the negotiations in a conference meeting Thursday.

• Romney told reporters after the meeting that McConnell is "fully behind the border bill" and "is not going to let political considerations of any campaign stand in the way of his support."

But there's lingering doubt that a deal will come to fruition amid persistent opposition from Senate and House conservatives.

• Senators have yet to see the actual bill text, which is expected in the coming days.
• "We have never had a package of this significance under consideration by the United States Congress," Sen. Kyrsten Sinema (I-Ariz.), one of the lead negotiators, told reporters. "And this is a unique moment. I think we should take it."

Meanwhile: Trump on Thursday called on "all willing states" to deploy their National Guards to help repel migrants at the border, as Abbott escalates his battle with the Biden administration over border enforcement authority.

• The governor has argued that Texas' right to defend itself from an "invasion" supersedes any federal statutes — defying Monday's Supreme Court ruling that allowed federal border officials to remove razor wire barriers set up by the state.
• 25 Republican governors have signed onto a joint statement supporting Abbott and Texas' "constitutional right to self-defense," including the use of razor wire fences and other tools.

Zoom out: As far as political issues go, immigration has the worst of all worlds. It is at once politically explosive, unimaginably complex policy-wise and easily challenged in court.

• Republicans — led by Trump — have made harsh border measures central to their political platform in recent years. Once fringe ideas, like deploying the military to deal with cartels, have become mainstream.
• If Republicans walk away from the bipartisan Senate deal, Biden will seek to paint them as putting partisan interests over real solutions to the border crisis.

The bottom line: The year of the border is just getting started.