President Biden took the border fight directly to former President Donald J. Trump on Tuesday, blaming his predecessor and putative challenger for torpedoing a bipartisan immigration agreement out of crass politics at the expense of national security.

Weighing in forcefully after months of largely staying out of the fray, Mr. Biden called on congressional Republicans to “show some spine” and stand up to Mr. Trump. But he effectively acknowledged that the deal negotiated over several months was doomed and vowed to make it a campaign issue against the opposition.

“All indications are this bill won’t even move forward to the Senate floor,” Mr. Biden said in a speech televised from the White House. “Why? A simple reason. Donald Trump. Because Donald Trump thinks this is bad for him politically.”

The president said that Mr. Trump would “rather weaponize this issue than actually solve it” and has leaned on Republicans to block it.

“It looks like they’re caving,” he added. “Frankly, they owe it to the American people to show some spine and do what they know to be right.”

The decision by Republicans to reject a bipartisan border deal that they had previously demanded not only paralyzed the immigration debate but also imperiled security assistance for Ukraine and Israel attached to the $118 billion measure, closing off what had been seen as the best remaining avenue to aid embattled American allies.