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Martin O’Malley, the former governor of Maryland who staked out early ground in the Democratic presidential race on the issue of reforming immigration laws, sent a letter to the Jeh Johnson, the secretary of the Department of Homeland Security, urging him to rethink planned raids of families who came over the border last year during a surge of crossings into the United States.

“If these reports are true, they raise serious questions about due process for people seeking asylum, and these raids promise to needlessly rip apart more families and communities,” Mr. O’Malley wrote.

“I am writing to respectfully request that D.H.S. reconsider this approach and instead look for ways to humanely treat people seeking refuge and to greatly improve the legal due process necessary for asylum,” he added. “When, in 2014, tens of thousands of children made the journey to escape death gangs in Honduras, El Salvador, and Guatemala, we called on Marylanders to step up and help accommodate them. Maryland took action to safeguard more refugee kids on a per-capita basis than any other state in the nation. Thousands of families opened their homes as foster families. We recruited pro-bono legal help for case preparation. And we did all of this at minimal cost to the taxpayer.”

Mr. O’Malley called such deportations a “last resort.”

The letter amounts to Mr. O’Malley maintaining pressure on the issue. Both he and Senator Bernie Sanders of Vermont, another candidate seeking the Democratic nomination, have criticized the deportation plan, which was recently revealed by The Washington Post.

Hillary Clinton, who served in the Obama administration, has walked a more careful line, with a spokeswoman issuing a statement saying she has “concerns” about the raids but treading carefully on criticizing federal officials.

Mr. O’Malley has struggled throughout the year to gain traction, and has just four weeks to make a dent before voting begins in the Iowa caucuses. But his role in the race, so far, has been to push Mrs. Clinton to the left on issues related to immigration as the Democratic Party has made Hispanics an important part of its emerging voter coalition.

The issue of immigration reform is not one that is always an easy fit for Mr. Sanders, who has suggested that there is a link between immigration and wage stagnation in the past. And Republicans, including but not limited to Donald J. Trump, have been criticized by groups focused on immigration for their comments about people coming to the country.

But for Mr. O’Malley, the issue has been at the forefront for the last few years. And when Mrs. Clinton, during the border surge, said at a town hall meeting with voters hosted by CNN that immigrant children should be sent “back” to be with their families, Mr. O’Malley was the only Democrat to take a differing view.

