Sen. Bernie Sanders (I-Vt.) penned a forceful letter to President Barack Obama on Thursday assailing the Department of Homeland Security's large-scale deportation raids targeting Central American immigrants, writing that the raids would condemn many migrants to a "death sentence" and calling on Obama to exercise his executive authority to protect vulnerable migrants.

The raids: Sanders' letter comes two weeks after the Washington Post reported that DHS was planning to conduct the nationwide raids, focusing on the wave of Central American migrants, many of them unaccompanied minors, who flocked to the United States in 2014 amid widespread gang violence in their home countries.

Homeland Security Secretary Jeh Johnson announced this week that authorities with the Immigrations and Customs Enforcement agency had rounded up 121 migrants in Georgia, Texas and North Carolina over the weekend. The Post reported that the migrants were being detained in federal facilities ahead of deportation to their countries of origin.

More raids are likely to follow. When announcing the raids on Monday, Johnson warned, "As I have said repeatedly, our borders are not open to illegal migration. If you come here illegally, we will send you back consistent with our laws and values."

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Sanders' letter: After saying he was "very disturbed" with the planned raids following the Post's report last month, Sanders, who is vying to become the Democratic presidential nominee, amped up his criticism in his letter Thursday.

"As a country, we have a fundamental responsibility to keep families together and welcome those seeking refuge from extreme violence and persecution," Sanders wrote the president. He added that the weekend raids flew in the face of Obama's "directive to 'more humanely' enforce our nation's immigration laws," referring to Obama's 2014 executive action extending deportation reprieves to up to 5 million undocumented immigrants.

The same relief should be extended to migrants targeted by the DHS raids, Sanders said. He decried barriers to effective legal counsel for migrants, urging the administration to guarantee them adequate representation as they fight deportations.

Framing the debate as one about basic moral principles, Sanders went on, "Raids are not the answer. We cannot continue to employ inhumane tactics involving rounding up and deporting tens of thousands of immigrant families to address a crisis that requires compassion."

But the deportation debate is more than a question of morality, Sanders' letter suggests; it's also about life and death. Referring to a recent Guardian investigation that found that as many as 83 deported migrants had been murdered in Honduras, Guatemala and El Salvador since 2014, Sanders implored the president "to immediately end these raids and not deport families back to countries where a death sentence awaits."

What other candidates say: DHS' actions sparked a heated political debate well before Sanders' letter Thursday. With differing degrees of intensity, Sanders' Democratic rivals, former Secretary of State Hillary Clinton and former Maryland Gov. Martin O'Malley, have also criticized the raids. Clinton's campaign told NBC News that she has "real concerns" about deporting migrants, adding that she wanted to see the legal system deal with the migrants in "a spirit of humanity and generosity."

O'Malley offered much more forthright criticism of the raids than Clinton did, tweeting on Christmas Eve, "We are a better nation than this."

For Clinton, who's seeking to woo Latino voters and prevent Sanders from gaining on her among nonwhites, the raids are a dicey issue. As the Central American migrant wave reached its peak in 2014, Clinton called for migrant children to be sent back to their home countries, arguing that that was the most compassionate policy.

"They should be sent back as soon as it can be determined who the responsible adults in their family are because there are concerns about whether all of them can be sent back, but I think all of them that can be should be reunited with their families," she said, as the Washington Times reported. "But we have to send a clear message that just because your child gets across the border, that doesn't mean your child gets to stay. We don't want to send a message that is contrary to our laws or encourage more children to make that dangerous journey."

Through a spokesman, Clinton said following this weekend's raids that the U.S. "should not be conducting large-scale raids and roundups that sow fear and division in our communities," Politico reported.

Meanwhile, the Democratic field's varying expressions of concern about the new raids contrasts sharply with the enthusiastic applause the raids received from Republican frontrunner Donald Trump, who claimed credit for the raids when the Post reported they would be carried out:

But as federal authorities embark on raids that may send migrants back to violence-plagued countries, Sanders' letter underscores that the administration won't be facing "pressure" only from anti-immigrant forces.