Julián Castro really wants to make sure that his fellow Democratic presidential candidates, and the voters watching the first 2020 Democratic presidential debate, know what “Section 1325” is.

It’s the section of Title 8 of the United States Code that makes it a misdemeanor for immigrants to enter the United States without papers.

Castro wants to get rid of it — so that being an unauthorized immigrant in the US would still be a civil offense but no longer a federal crime.

And he’s pushing the rest of the Democratic field to join him.

Elizabeth Warren already has endorsed repeal of the “illegal entry” provision. Even moderate Tim Ryan implied he’d be open to repeal during Wednesday’s debate. Beto O’Rourke, who has an aggressive immigration plan of his own, was the only candidate who refused.

In a Democratic primary that has shown the party has shifted leftward on several issues since the Obama administration, this exchange was still remarkable. In fiscal year 2016, immigration offenses — illegal entry and reentry chief among them — made up a majority of federal criminal prosecutions. In 2019, as a result of Castro’s hectoring on the debate stage, the Democratic presidential field debated, for several minutes, whether it should be a crime at all.

Illegal entry has been a crime for 90 years, but only recently has prosecution for it become common

Decriminalizing migration isn’t exactly the same as opening the borders. People coming to the US without papers could still be deported if they were caught and brought before an immigration judge. But it would make unauthorized immigration purely a civil offense, instead of a criminal one.

The distinction matters a lot. Criminal prosecution of illegal entry was what gave the Trump administration the power to separate thousands of families in 2018. It referred thousands of parents for criminal prosecution for illegal entry — advertised as a “zero tolerance” approach — and thus separated them from their children to send them to criminal custody.

If you’re an unauthorized immigrant in the US, you’re committing a civil violation: being present in the US without a valid immigration status. That’s breaking a law, but it’s not a crime, in the same way that violating the speed limit isn’t a crime. If you’re arrested, you can be deported — a huge change to an immigrant’s life, but not technically a criminal punishment.

But if you cross the US-Mexico border between ports of entry without papers, you are committing a federal misdemeanor: illegal entry. And you can be jailed and fined in addition to getting deported.

In one respect, this system has been on the books since 1929, when illegal entry was first made a misdemeanor. But for most of the 20th century, it was kind of irrelevant. Most people who came into the US without papers weren’t tracked down and deported. Presidents generally decided that it wasn’t worth it to spend US attorneys’ time prosecuting endless misdemeanor illegal entry cases. Those who were caught crossing the border were generally informally returned.

Under the Bush administration, however, as an independent immigration enforcement system began to develop and mature, both civil immigration cases (in separate immigration courts) and widespread criminal illegal entry prosecutions became common.

The result swamped federal criminal courts along the border. For the past several years, immigration offenses — illegal entry and reentry — have been the most common crimes for which people are convicted in US federal criminal courts. (In fiscal year 2016, immigration offenses made up a majority of all federal criminal prosecutions.) And the courts along the border where entrants are prosecuted are routinely the busiest in the country.

More recently, the Trump administration’s attempts at “zero tolerance” prosecution of illegal entry were the legal basis for its widespread separation of families in 2018: Children were separated because their parents were being transferred to criminal custody for prosecution.

Castro’s proposal isn’t quite “open borders” — but President Trump is likely to describe it that way

Under the Castro proposal, the only plan any candidate has put out that fleshes out the idea, family separation wouldn’t be possible because criminal prosecution wouldn’t be possible.

Under that plan, an immigrant crossing into the US without papers, whether he was seeking asylum in the US or coming for some other reason, would not be committing a federal crime. If caught by Border Patrol agents, she’d be detained for a brief amount of time, but if she didn’t raise any red flags, she’d be released into the US (with a case management system to check up on her whereabouts) pending an immigration hearing. If she didn’t qualify for some form of legal status like asylum, she’d still ultimately be ordered deported from the US.

Proposing that illegal entry no longer be a federal crime is the policy equivalent of the “no human is illegal” slogan — a way to combat hawkish attitudes toward the “rule of law” by challenging the idea that migration ought to be a matter of crime and punishment to begin with. But it’s also a key justification for reversing the past few decades of border crackdown, by unpinning immigration enforcement — at least when it comes to unauthorized immigrants themselves — from crime.

Both of those draw strong contrasts not only with President Trump but with the trends in enforcement that preceded Trump (many of which peaked under President Obama).

Trump is highly likely to describe whoever faces him in the 2020 general election as “weak” on the borders and immigration. It’s been his single most common attack on Democrats as president.

The question is whether, if the eventual Democratic nominee has endorsed repealing Section 1325, the American people will agree.