Opposition to the agreement is widespread in Guatemala, where the Constitutional Court ruled recently that the Guatemalan government needed congressional approval to make a safe third country deal with the United States. That ruling, which came amid negotiations between the Trump administration and the administration of the Guatemalan president, Jimmy Morales, prompted Mr. Trump to threaten the Central American nation with punitive tariffs, a travel ban and taxes on the remittances sent home by Guatemalan migrants in the United States.

On Friday, Mr. Morales seemed to be trying to skirt the court ruling by avoiding the use of the term “safe third country” in his statement on the deal. But the Trump administration did use the term, giving impetus to potential legal challenges in Guatemala.

There are also several steps the United States government must take before the agreement can be put into effect.

The Justice Department and the Department of Homeland Security would have to certify that Guatemala has a “full and fair” asylum system, and is able to protect asylum seekers from other countries if the United States sends them there.

The logistics of the plan are also still being determined. What would be the process for determining that a migrant should be returned to Guatemala? How many migrants could be sent back each week? How would they be transported?

A draft of the plan circulating among American officials suggests that initially, a limited number of migrants would be sent to Guatemala each week — perhaps several hundred in a month. That would be a tiny fraction of the tens of thousands that have been seeking asylum each month at the border with Mexico.