The push underscores the massive resistance the White House faces for the policy of splitting up families crossing U.S. borders illegally. While backlash has been bipartisan, the most fierce resistance has come from the political left in blue pockets of the country. Most of the members of Congress calling for the Cabinet member's ouster represent those areas.
Sen. Kamala Harris, D-Calif., first called for Nielsen to step down over the widely condemned separation practice. In a tweet Monday, she contended that "under [Nielsen's] watch, our government has committed human rights abuses by breaking up families along the southern border."
"And she has failed to be accountable to and transparent with the American people," said the first-term senator, who is considered a potential 2020 Democratic presidential candidate.
The highest-ranking Democrat to call for Nielsen's resignation is House Minority Leader Nancy Pelosi. Other Democratic lawmakers who have pushed for the secretary to step down include Sens. Mazie Hirono of Hawaii, Jeff Merkley of Oregon, Tina Smith of Minnesota and Richard Blumenthal of Connecticut, and Reps. Ted Lieu and Barbara Lee of California, Kathleen Rice of New York and Donald Payne of New Jersey.
President Donald Trump's "zero tolerance" policy — which criminally prosecutes all migrants caught crossing U.S. borders illegally — has resulted in the separation of children from parents. Condemnation of the practice has increased in recent days as photos surfaced of children sleeping on the floor within wire cages. Other photos and audio from within a detention center depict wailing children.
As she leads the agency that oversees border security, Nielsen has become one of the public faces of the crisis. Democrats have argued that she has misdirected or outright lied to obfuscate the Trump administration practices. Nielsen first drew broad condemnation Sunday when she falsely claimed that "we do not have a policy of separating families at the border. Period."
Lawmakers, including many Republicans, have noted that the Trump administration has the ability to change the policy immediately. Numerous members of Congress have urged the president to abandon the practice. However, Nielsen falsely claimed on Monday that "Congress and the court created this problem, and Congress alone can fix it."
The Trump administration has held out on a fix while seeking broader immigration legislation that would fund Trump's proposed border wall and enact stricter limits on legal immigration. On Monday, Nielsen denied that children are being used as a political "pawn," saying "we are trying to protect the children."
In a statement Monday afternoon, DHS press secretary Tyler Houlton said Nielsen is carrying out the job she was appointed and confirmed to do.
"Instead of criticizing a government official who is actually doing the job she was nominated, confirmed & sworn to do and enforcing the laws passed by Congress, the obstructionists in Congress should get to work to secure our borders, end legal loopholes & protect American lives," he said in a tweeted statement.
Despite what DHS and the White House have argued, the family separation policy is not a law passed by Congress.
None of the Democrats who called for Nielsen to step down this week supported her confirmation in December. Smith, appointed to her seat following Sen. Al Franken's resignation, was not yet in office.
Nielsen already came close to resigning once last month, according to The New York Times. Trump reportedly lashed out at her in front of his entire Cabinet for failing to adequately protect U.S. borders, according to the newspaper.
At the time, Trump believed Nielsen was resisting his push to separate children from parents, the newspaper reported.