Department of Homeland Security (DHS) Secretary Kirstjen Nielsen is expected to face fierce questioning from House Democrats about her role in spearheading the implementation of the Trump administration's hardline immigration agenda at a public hearing Wednesday morning. 

Nielsen, who has managed to keep her job despite reports of rifts with President Trump, will testify before the House Homeland Security Committee, led by Democratic Rep. Bennie Thompson of Mississippi. Thompson had threatened to subpoena Nielsen before she agreed to appear voluntarily. 

The main line of questioning from the committee's Democratic majority will center on Mr. Trump's controversial national emergency declaration, his proposed border wall, the discontinued family-separation policy and migrant children who have died in U.S. custody, two congressional aides told CBS News.  

Although some members of the committee will likely delve into other immigration-related issues, the aides said the hearing will largely focus on border security and Nielsen's oversight of the administration's stringent enforcement strategy. 

Nielsen's high-profile testimony comes as the president continues to defend his national emergency proclamation which allows him to access billions of dollars from military construction funds to build a border wall. The move is being challenged in court through a multi-state lawsuit and in Congress through a resolution of disapproval that has passed the House.

Mr. Trump, Nielsen and other administration officials have advocated for stricter immigration enforcement and the construction of border barriers by pointing to what they believe is a humanitarian and national security crisis along the southwestern border. On Tuesday, Customs and Border Protection commissioner Kevin McAleenan said the situation remained at a "breaking point" for U.S. officials, citing a significant surge in the number of migrant families apprehended near the border in February.

The Homeland Security Committee hearing is one of four simultaneous hearings in which congressional Democrats will get a chance to scrutinize the Trump administration's hardline immigration policies. 

McAleenan is scheduled to testify before the Senate Judiciary Committee and the DHS inspector general will appear before the House Appropriations Committee. Dozens of Deferred Action for Childhood Arrivals (DACA) and Temporary Protected Status (TPS) recipients will appear before the House Judiciary Committee to detail their experiences under the programs, which the administration is seeking to terminate.