Thousands of migrants have crossed into the United States in recent days, from California to Texas, with many more still arriving by bus and cargo trains to Mexican border towns on the heels of record migration flows further south.

The dramatic increase along the border – notably in San Diego, California, and the Texan cities of El Paso and Eagle Pass – marks a turning point after numbers had plummeted in recent months, and could create fresh political challenges for U.S. President Joe Biden heading into election season.

Biden in May rolled out a new policy to deter illegal crossings, including deporting migrants and banning re-entry for five years, as his administration grappled with migration at record highs.

Within a month the tougher measures drove the border-crossing rate down some 70%.

But a recent uptick in arrivals at the border, combined with vastly higher numbers of people on their way north across Central and South America and riding dangerous cargo trains through Mexico, suggest the early deterrent effect is wearing off.

Experts say the U.S. lacks the capacity to detain and process migrants at the border, often making it impossible for the administration to carry out the harsh penalties it announced in May.

As a result, some asylum seekers who cross illegally are being released into the U.S. with a future court date, rather than being deported – becoming success stories repeated back to migrants still en route.

“The (Biden administration) hit on a smart strategy, but they don’t have the resources or capacity to implement it,” said Andrew Selee, head of the Migration Policy Institute.

In response to questions from Reuters, U.S. Customs and Border Protection (CBP) said it was “safely and efficiently” processing migrants, and would impose consequences, including deportation, on migrants without a legal basis to stay in the country.

Mexican President Andres Manuel Lopez Obrador on Thursday derided the lack of an international plan to help countries lift their citizens out of poverty and thus avoid a key migration driver. He praised Biden for creating legal pathways for migrants but said they needed to be expanded.