The Biden administration will toughen an asylum ban at the U.S.-Mexico border to keep it in place for longer, U.S. Department of Homeland Security officials said on Monday, signaling a desire to further curb illegal crossings.

The change, effective just after midnight, will leave asylum restrictions in place until arrests of migrants crossing illegally drop below a daily average of 1,500 over 28 days, lengthened from the current seven-day period, one of the officials said on a call with reporters.

President Joe Biden, a Democrat, issued the asylum ban in June to drive down record numbers of migrants caught crossing illegally. Immigration is a top voter issue in the run-up to the Nov. 5 election, which is pitting Vice President Kamala Harris against Republican Donald Trump, an immigration hardliner.

Harris backs making the ban even harder to lift, Reuters reported last week, but the Biden administration did not adopt her proposal.

U.S. border authorities have apprehended roughly 54,000 migrants in September to date, down steeply from a peak of 250,000 in December, a DHS official said.

A daily average of 1,500 over 28 days would represent a total of 42,000 migrants in that period.

As part of the changes to the asylum ban, all unaccompanied children caught crossing illegally will be counted in the tally used to decide whether the restrictions can be lifted. Previously, only children from Mexico and Canada were counted.

The stricter approach "ensures that the drop in encounters is a sustained decrease" and not tied to short-term trends, a DHS official said, speaking under condition of anonymity.

Immigrant rights groups led by the American Civil Liberties Union have sued over the asylum ban, arguing that it runs counter to U.S. asylum law and closely parallels a Trump ban blocked in court.