The House Judiciary Committee has secured the testimony of President Trump Donald John TrumpFederal prosecutor speaks out, says Barr 'has brought shame' on Justice Dept. Former Pence aide: White House staffers discussed Trump refusing to leave office Progressive group buys domain name of Trump's No. 1 Supreme Court pick MORE's former campaign manager Corey Lewandowski Corey R. LewandowskiTrump faces tricky choice on Supreme Court pick How Trump can win reelection: Focus on Democrats, not himself Trump Jr. distances from Bannon group, says he attended 'single' event MORE later this month, marking the first public appearance of a Trump ally as Democrats seek to investigate possible obstruction by the president.

Judiciary Chairman Jerrold Nadler Jerrold (Jerry) Lewis NadlerDemocrats shoot down talk of expanding Supreme Court Schumer: 'Nothing is off the table' if GOP moves forward with Ginsburg replacement Top Democrats call for DOJ watchdog to probe Barr over possible 2020 election influence MORE (D-N.Y.), who subpoenaed Lewandowski to testify by Sept. 17, announced on Monday that he would appear on that date.

Nadler has also subpoenaed former White House officials Rob Porter and Rick Dearborn to testify with Lewandowski at a joint hearing, noting that all three are tied to a key episode of possible obstruction by Trump described in former special counsel Robert Mueller Robert (Bob) MuellerCNN's Toobin warns McCabe is in 'perilous condition' with emboldened Trump CNN anchor rips Trump over Stone while evoking Clinton-Lynch tarmac meeting The Hill's 12:30 Report: New Hampshire fallout MORE’s report.

But it is unclear if Porter and Dearborn will appear, since the White House has successfully blocked both former and current officials from testifying publicly up until this point.

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The White House has argued that confidentiality protections offered to the executive branch apply to current and former aides. This effectively prevents them from disclosing what took place during their time in the administration, the White House contends.

While both Republican and Democratic administrations have invoked the immunity concept, legal experts say there is virtually no case law on the subject. Some predict the White House is likely to lose if challenged in court.

Lewandowski, who never served in the Trump administration and therefore is outside the White House’s current claims of immunity, publicly stated that he would testify before the committee, vowing he would do so with the goal of defending the president.

“I am an open book. I want to go and remind the American people that these guys are on a witch hunt, right?” Lewandowski, who is considering a New Hampshire Senate run, told Fox News Radio last month.