The immigration issue already had a noticeable impact on the race during the first round of Democratic debates in June. On the first night, Mr. Castro pointedly asked the other candidates to join him in supporting the repeal of a statute that makes crossing the border without permission a criminal offense. The next night, with another 10 presidential candidates onstage, moderators asked for a show of hands of those who agreed with Mr. Castro and supported decriminalizing the offense. Eight hands rose. Some candidates added that they also supported providing undocumented immigrants with health care.

The next day, Mr. Trump took advantage. “That’s the end of that race!” he tweeted.

Mr. Biden, the early front-runner for the Democratic nomination, has spent most of his time focusing on Mr. Trump’s policies through campaign videos, statements and essays. In an opinion piece in The Miami Herald, Mr. Biden targeted Mr. Trump’s threat to deport millions of immigrants and restrict temporary protections for migrants fleeing disasters. He also blamed Mr. Trump for damaging the United States’ relationship with Mexico.

Mr. Biden has made clear that he would invest in Central American countries and technology at the border and support Dreamers. But he has released few details on whom he would prioritize deporting and how he would eliminate the immigration court backlog.

The Biden campaign said in a statement that the candidate is focused on addressing the root causes of migration and would, as president, give legal status to Dreamers and streamline the asylum process. The campaign had harsh words for Mr. Trump’s “repugnant” treatment of migrant children and immigration policies, which it described as “contrary to our values as a nation.”

The Democratic candidates have all said they would end the Trump administration’s practices of limiting who can apply for asylum at ports of entry and returning migrants to Mexico while their cases are processed. They would all also increase the annual cap on refugees allowed into the United States, which the president has lowered significantly, and end both the administration’s travel ban and its policy of separating families at the border.

“The Democrats will run on a lesser-of-two-evils strategy as they have been for many years,” Mr. Newman predicted. “There’s never been an easier time for them to do that.”