The Washington Post
reported late Wednesday that the nationwide raids will focus on hundreds of families that have entered the United States since early last year. The plan is the first large-scale effort to deport families that have fled violence in Central America, the Post reported.
Sanders said in a statement he was "disappointed" by the plan.
"As we spend time with our families this holiday season, we who are parents should ask ourselves what we would do if our children faced the danger and violence these children do? How far would we go to protect them?" he said. "We need to take steps to protect children and families seeking refuge here, not cast them out."
O'Malley, who is running well behind Sanders and Democratic presidential front-runner Hillary Clinton, said the plan is "completely at odds with our character as a nation."
"The world is watching, and it is up to us to decide whether we want to live up to our values, or whether we are ready to shrug them off and turn our backs on those most in need," O'Malley said.
The former Maryland governor also fired off a series of tweets, hitting his favorite target on the campaign trail, Donald Trump, the Republican front-runner who has called for the deportation of the estimated 11 million undocumented immigrants living in the United States.
"A Christmas Refugee Roundup sounds like something @realDonaldTrump would concoct. Remember: Jesus was a refugee child who fled death gangs," O'Malley said.
Later Thursday, Xochitl Hinojosa, a spokeswoman for Clinton's campaign, said the former secretary of state has "real concerns about these reports."
"She believes it is critical that everyone has a full and fair hearing, and that our country provides refuge to those that need it," Hinojosa told CNN in a statement. "And we should be guided by a spirit of humanity and generosity as we approach these issues."
Clinton took a more conservative position
in a CNN town hall event last year on the subject of how to deal with unaccompanied children who enter the United States illegally, calling for them to be sent back to their homelands.
"We have to send a clear message, just because your child gets across the border, that doesn't mean the child gets to stay. So, we don't want to send a message that is contrary to our laws or will encourage more children to make that dangerous journey," she said then. But she added that she wanted the executive branch to have more leeway.
"We need to show humanity with respect to people who are working, contributing right now," she said. "And deporting them, leaving their children alone or deporting an adolescent, doing anything that is so contrary to our core values, just makes no sense."
The deportations could begin as soon as early January, according to the Post. A spokesman for the Department of Homeland Security told CNN in a statement late Wednesday that Immigration and Customs Enforcement "focuses on individuals who pose a threat to national security, public safety and border security."
The statement added that ICE removed or returned 235,413 people over the last fiscal year.
"Our border is not open to illegal immigration, and if individuals come here illegally, do not qualify for asylum or other relief, and have final orders of removal, they will be sent back consistent with our ideas and our values," the statement read.
Immigration has emerged as a key issue in the 2016 presidential race, particularly among Republicans, whose front-runner, Trump, has labeled some undocumented immigrants as "rapists" and called for the construction of a wall along the southern border, a plan embraced by many Republicans and repudiated by Democratic candidates.
Trump tweeted his support of the proposed raids Thursday, saying they were happening because of the pressure he put forth on the issue.
Some Democrats, however, have been critical of the White House over deportations and have tried to portray themselves as more open to accepting undocumented immigrants than President Barack Obama. Republican candidates have overwhelmingly prioritized border security over granting legal status to those who came here illegally.