The debate then turned to immigration and whether crossing the border should be decriminalized. Ms. Warren and Beto O’Rourke argued for fully decriminalizing border crossing, while Mr. Buttigieg was more cautious.

Of moderate Democrats, Ms. Warren said: “I don’t understand why anybody goes to all the trouble of running for president of the United States to talk about what we really can’t do and shouldn’t fight for.”

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More 2020 news: A new California law requires President Trump to release his tax returns to get on the primary ballot. It is likely to face legal challenges.