REYNOSA, Mexico — An estimated 15,000 mostly Haitian immigrants have packed into every crevice of this northern Mexican city on the Rio Grande across from McAllen, Texas, and won’t leave. At least not until after 11:59 p.m. on May 11.

That’s the moment when the pandemic-era “Title 42” rapid expulsion policy finally expires and is replaced by a new, untested Biden administration plan for keeping them in Reynosa as well as the “volatile logjam,” as The New York Times recently termed them, of tens of thousands of other immigrants now waiting for the policy change all over Mexico.

But because of a powerful deterrent of Title 42 fairly unique to them, the 15,000 Haitians in Reynosa make for a good bellwether for if the administration’s replacement strategy will hold them back as Title 42 has, or will invite the most chaotic rush on the U.S. southern border yet in this third long year of the most voluminous mass migration event in recorded American history. …