Unauthorized entries are swelling in defiance of the lockdown President Trump imposed on the border during the pandemic and shaping up as the first significant challenge to President-elect Joseph R. Biden Jr.’s pledge to adopt a more compassionate policy along America’s 1,100-mile border with Mexico.

After a steep decline in border crossings through much of this year, interceptions of unauthorized migrants along the Arizona-Mexico border are climbing again: Detentions in October were up 30 percent over September, and the figure in coming months is expected be even higher, despite the biting cold in the Sonoran desert.

The rising numbers suggest that the Trump administration’s expulsion policy, an emergency measure to halt spread of the coronavirus, is encouraging migrants to make repeated tries, in ever-more-remote locations, until they succeed in crossing the frontier undetected.

And they are likely the leading edge of a much more substantial surge toward the border, immigration analysts say, as a worsening economy in Central America, the disaster wrought by Hurricanes Eta and Iota and expectations of a more lenient U.S. border policy drive ever-larger numbers toward the United States.