WASHINGTON (AP) — The Biden administration is scrambling to manage a growing humanitarian and political challenge at the U.S.-Mexico border that threatens to overshadow its ambitious legislative agenda.

With the number of migrants surging, administration officials say Biden inherited an untenable situation that resulted from what they say was President Donald Trump’s undermining and weakening of the immigration system.

But as Congress pivots to immigration legislation, stories of unaccompanied minors and families trying to cross border have begun to dominate the headlines, distracting from the...