The Democratic candidates are now debating decriminalizing crossing the border.

Some background on the law: Right now anyone who crosses the border illegally can be charged with a misdemeanor.

Part of the Immigration and Nationality Act, known as Section 1325, says illegally entering the United States can be prosecuted as a misdemeanor. It's punishable by up to six months in prison. And it's become one of the most prosecuted federal crimes in the United States.

Section 1325 has been on the books for decades. But for many years it wasn't often enforced. That notably changed in 2005, when President George W. Bush's administration implemented what was known as "Operation Streamline," increasing criminal prosecutions at the border in an effort to deter illegal immigration.

According to an analysis from the Transactional Records Access Clearinghouse at Syracuse University, federal prosecutors charged more than 52,000 people with illegal entry under Section 1325 in the first eight months of the 2019 fiscal year, which began in October.

Supporters say that provision of the law isn't necessary and was used to separate families. Castro and others who argue that Section 1325 should be repealed -- including Sen. Elizabeth Warren and South Bend Mayor Pete Buttigieg -- say that it isn't necessary given that crossing the border illegally is already a civil offense that can result in deportation.

They also argue that Section 1325 made many of the Trump administration's family separations at the border possible. Under the "zero tolerance" policy, children were separated when their parents were criminally prosecuted for illegally crossing the border.