Senator Elizabeth Warren and other Democrats expressed satisfaction at the news that Kirstjen Nielsen was resigning her post as Homeland Security secretary effective on Wednesday.

‘[Nielsen] separated mamas from their babies and locked little children in cages,’ the Massachusetts senator tweeted on Sunday.

‘I’m not sorry to see her go. I only wish she’d been fired long ago, before she ever had a chance to resign.’

House Speaker Nancy Pelosi released a statement saying: ‘It is deeply alarming that the Trump administration official who put children in cages is reportedly resigning because she is not extreme enough for the White House’s liking.

‘[Nielsen] separated mamas from their babies and locked little children in cages,’ Elizabeth Warren, the Massachusetts senator, tweeted on Sunday.

‘I’m not sorry to see her go,' Warren tweeted on Sunday. 'I only wish she’d been fired long ago, before she ever had a chance to resign.’

‘The president’s dangerous and cruel anti-immigrant policies have only worsened the humanitarian suffering at the border and inflicted vast suffering on the families who have been torn apart.’

House Homeland Security Committee Chairman Bennie Thompson, D-Miss., said her tenure was 'a disaster from the start.'

The policies she helped create 'have been an abysmal failure and have helped create the humanitarian crisis at the border.'

His Senate counterpart, Sen. Ron Johnson, R-Wisc., said the government needed 'steady, informed and effective leadership in the administration and in Congress to have any hope of fixing our out-of-control border security and immigration problems.'

Senator Richard Blumenthal of Connecticut tweeted: ‘Secretary Nielsen leaves a legacy of separating thousands of families, caging children, & denying it happened.

‘We will be grappling with the damage she has wreaked for decades to come.’

Senate Minority Leader Chuck Schumer tweeted: ‘When even the most radical voices in the administration aren’t radical enough for President Trump, you know he’s completely lost touch with the American people.’

Senator Mazie Hirono of Hawaii was scathing in her critique of Nielsen.

She tweeted: ‘I have repeatedly called for Kirstjen Nielsen's resignation for her lies in service of [Trump’s] abhorrent immigration policies, and I welcome her decision today.

‘History will not judge her or the president kindly.’

Julian Castro, the former secretary of housing and urban development under Barack Obama, tweeted: ‘Sec. Nielsen has been a willing partner in the most cruel and short-sighted immigration agenda in decades.

Senator Richard Blumenthal of Connecticut tweeted: ‘The Nielsen era was a disgrace, but we should all fear what comes next'

Julian Castro, the former secretary of housing and urban development under Barack Obama, tweeted: ‘Sec. Nielsen has been a willing partner in the most cruel and short-sighted immigration agenda in decades'

House Rep. Adam Schiff tweeted: 'Secretary Nielsen’s policies were cruel and unconscionable. A generation from now, we will still recoil at how America separated children from their parents and put them in cages'

Senate Minority Leader Chuck Schumer tweeted: ‘When even the most radical voices in the administration aren’t radical enough for President Trump, you know he’s completely lost touch with the American people.’

Senator Mazie Hirono was scathing. She tweeted: ‘I have repeatedly called for Kirstjen Nielsen's resignation for her lies in service of [Trump’s] abhorrent immigration policies, and I welcome her decision today. History will not judge her or the president kindly’

Senator Kamala Harris of California tweeted: 'Kirstjen Nielsen misled the American people and defended Trump's inhumane policy of separating children from their parents. It was long past time for her to go.'

House Speaker Nancy Pelosi released a statement saying: ‘It is deeply alarming that the Trump administration official who put children in cages is reportedly resigning because she is not extreme enough for the White House’s liking'

‘We must defeat Donald Trump and create a more compassionate and sensible immigration system that puts people first.’

House Rep. Adam Schiff tweeted: 'Secretary Nielsen’s policies were cruel and unconscionable.

'A generation from now, we will still recoil at how America separated children from their parents and put them in cages.

'But make no mistake, a change in personnel is pointless - the problem comes from the very top.'

Nielsen initially said her resignation would take effect immediately, but she later tweeted on Sunday that she would stay on until Wednesday 'to assist with an orderly transition'

Senator Kamala Harris of California tweeted: 'Kirstjen Nielsen misled the American people and defended Trump's inhumane policy of separating children from their parents.

'It was long past time for her to go.'

Nielsen resigned on Sunday amid the administration's growing frustration and bitterness over the number of Central American families crossing the southern border.

President Trump announced on Sunday in a tweet that U.S. Customs and Border Protection Commissioner Kevin McAleenan would be taking over as acting head of the department.

McAleenan is a longtime border official who is well-respected by members of Congress and within the administration.

The decision to name a top immigration officer to the post reflects Trump's priority for the sprawling department founded to combat terrorism following the September 11, 2001 attacks.

Nielsen defended the Trump administration's policy of separating migrant children from their parents. The image above shows a young girl walking along a stretch of border fence after illegally crossing the Rio Grande near Penitas, Texas on Saturday

Though Trump aides were eyeing a staff shake-up at Homeland Security and had already withdrawn the nomination for another key immigration post, the development Sunday was unexpected.

Nielsen traveled to the U.S.-Mexico border on Friday with Trump to participate in a roundtable with border officers and local law enforcement.

There she echoed Trump's comments on the situation at the border, though she ducked out of the room without explanation for some time while Trump spoke.

As they toured a section of newly rebuilt barriers, Nielsen was at Trump's side, introducing him to local officials.

She returned to Washington afterward on a Coast Guard Gulfstream, as Trump continued on a fundraising trip to California and Nevada.

Nielsen had grown increasingly frustrated by what she saw as a lack of support from other departments and increased meddling by Trump aides on difficult immigration issues, according to three people familiar with details of her resignation.

They spoke on condition of anonymity because they were not authorized to discuss the matter.

She went into the White House on Sunday to meet with Trump not knowing whether she'd be fired or would resign.

She ended up resigning, though she was not forced to do so, they said.

Nielsen is the latest person felled in the Trump administration's unprecedented churn of top staff and Cabinet officials, brought about by the president's mercurial management style, insistence on blind loyalty and rash policy announcements.

Nielsen was also the highest profile female Cabinet member, and her exit leaves DHS along with the Pentagon and the White House staff itself without permanent heads.

Patrick Shanahan has held the post of acting defense secretary since the former secretary, Jim Mattis, was pushed out in December over criticism of the president's Syria withdrawal plans.

Acting White House chief of staff Mick Mulvaney has held his post since January, following John Kelly's resignation last year.

Her resignation later lacked any sense of controversy - unlike those of others who have left.

'Despite our progress in reforming homeland security for a new age, I have determined that it is the right time for me to step aside,' she wrote.

Department of Homeland Security Secretary Kirstjen Nielsen met with President Donald Trump Sunday evening and submitted her resignation

Nielsen was with President Trump when he visited the border on Friday

U.S. Customs and Border Protection Commissioner Kevin K. McAleenan will be acting Department of Homeland Security secretary

'I hope that the next secretary will have the support of Congress and the courts in fixing the laws which have impeded our ability to fully secure America's borders and which have contributed to discord in our nation's discourse.'

Her replacement, McAleenan, has helped shape many of the administration's policies to date and is considered highly competent by congressional leaders, the White House and Homeland Security officials.

But it's unclear if he can have much more of an effect on the issues at the border.

The Trump administration has bumped up against legal restrictions and court rulings that have hamstrung many of its major efforts to remake border security.

Tensions between the White House and Nielsen have persisted almost from the moment she became secretary, after her predecessor, Kelly, became the White House chief of staff in 2017.

Nielsen was viewed as resistant to some of the harshest immigration measures supported by the president and his aides, particularly senior adviser Stephen Miller, both on matters around the border and others like protected status for some refugees.

Once Kelly left the White House, Nielsen's days appeared to be numbered.

She had expected to be pushed out last November, but her exit never materialized.

During the government shutdown over Trump's insistence for funding for a border wall, Nielsen's standing inside the White House even appeared to rise.

Hours after announcing Nielsen's resignation, President Trump on Sunday took credit for the large number of arrests at the border while threatening to close the frontier with Mexico if Democrats in Congress do not agree to reform the immigration system

In a second tweet, Trump continued: ‘Mexico must apprehend all illegals and not let them make the long march up to the United States, or we will have no other choice than to Close the Border and/or institute Tariffs. Our Country is FULL!’

But in recent weeks, as a new wave of migration has taxed resources along the border and as Trump sought to regain control of the issue for his 2020 re-election campaign, tensions flared anew.

Kevin McAleenan Kevin McAleenan is the child of immigrants. His mom is from Finland and his father is from Ireland. He has served as the commissioner of U.S. Customs and Border Protection since January 2017. He was confirmed by the U.S. Senate on March 19, 2018, by a vote of 77–19. His confirmation was delayed for three months over allegations from anonymous accusers that he had an affair with a subordinate and bypassed proper channels to fund an immigration detention center. An inspector general investigation cleared him of all wrongdoing. Prior to becoming acting Commissioner of U.S. Customs and Border Protection, he served as Deputy Commissioner during the Obama administration and had other positions in the agency. McAleenan has also served as the Area Port Director of Los Angeles International Airport. Her received his Juris Doctor Degree from the University of Chicago Law School. Advertisement

The final straw came when Trump gave Nielsen no heads-up or opportunity to discuss his decision to pull the nomination of acting Immigration and Customs Enforcement Director Ron Vitiello - a move seen as part of a larger effort by Miller, an immigration hardliner, and his allies at the White House to clean house at the department and bring in more people who share their views, the people said.

Nielsen had wanted to discuss the move with Trump during their visit to the border Friday, but when there was no time, she asked for the meeting Sunday.

She walked into it prepared to resign, depending on what she heard.

The people described mounting frustrations on both sides, with Trump exasperated at the situation at the border and Nielsen frustrated by White House actions she felt were counterproductive.

Arrests along the southern border have skyrocketed recently.

Border agents are on track to make 100,000 arrests and denials of entry at the southern border in March, over half of which are families with children.

A press conference to announce the most recent border numbers - scheduled to be held by McAleenan on Monday - was postponed.

Nielsen dutifully pushed Trump's immigration policies, including funding for his border wall, and defended the administration's practice of separating children from parents.

She told a Senate committee that removing children from parents facing criminal charges happens 'in the United States every day.'

But she was also instrumental in ending the policy.

Under Nielsen, migrants seeking asylum are waiting in Mexico as their cases progress.

She also moved to abandon longstanding regulations that dictate how long children are allowed to be held in immigration detention, and requested bed space from the U.S. military for some 12,000 people in an effort to detain all families who cross the border.

Right now there is space for about 3,000 families, and facilities are at capacity.

She also advocated for strong cybersecurity defense and often said she believed the next major terror attack would occur online - not by planes or bombs.

Nielsen reportedly was not given a head's up on Thursday when the White House retracted its nomination of Ron Vitiello to lead U.S. Immigration and Customs Enforcement

She was tasked with helping states secure elections following Russian interference during the 2018 election.

She led the federal agency since December 2017, and was this administration's third Homeland Security secretary.

A protege of Kelly's, he brought her to the White House after Trump named him chief of staff. She earned a reputation as an enforcer working to corral the chaotic West Wing.

Nielsen, 45, previously served as a special assistant to President George W. Bush and worked for the Transportation Security Administration.

She rose through the Trump ranks quickly - joining the transition team after the election to help guide Kelly through the confirmation process.

She quickly became a trusted aide to Kelly, and the two worked together to impose order on a dysfunctional White House that lacked clear lines of command.