The head of the House Judiciary Committee on Monday subpoenaed Rob Porter for testimony, making the former White House staff secretary the latest witness to be ordered to Capitol Hill as Democrats seek to investigate possible episodes of obstruction of justice by 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.

Committee 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.) is seeking to have Porter testify on Sept. 17, the same day the committee wants to hear from former Trump 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 and former White House official Rick Dearborn.

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“Rob Porter was prominently featured in the Special Counsel’s description of President Trump’s efforts to obstruct justice by directing then-White House Counsel Don McGahn to fire the Special Counsel, and then ordering him to lie about it," Nadler said in a statement.

According to former special counsel Robert Mueller's report, the president ordered Porter to ask then-White House counsel Don McGahn to create a misleading record that claimed Trump never asked McGahn to remove Mueller, who was investigating Russia's interference in the 2016 election and possible obstruction of the investigation by Trump.

Trump directed McGahn to have Mueller removed, according to the report, but McGhan refused to follow through with the order, instead drafting a resignation letter that he ultimately did not submit. Trump then reportedly attempted to change the narrative by ordering Porter to create a record refuting McGahn's account.

“The President told Porter that the article was ‘bullshit’ and he had not sought to terminate the Special Counsel. The President said that McGahn leaked to the media to make himself look good," according to the 448-page report.

"The President then directed Porter to tell McGahn to create a record to make clear that the President never directed McGahn to fire the Special Counsel. Porter thought the matter should be handled by the White House communications office, but the President said he wanted McGahn to write a letter to the file ‘for our records’ and wanted something beyond a press statement to demonstrate that the reporting was inaccurate."

While Mueller did not find sufficient evidence to conclude that the Trump campaign coordinated with Russia, he did not make a determination as to whether the president obstructed justice. Attorney General William Barr and other Department of Justice officials ultimately decided that the evidence laid out in the Mueller report did not reach the threshold needed to charge Trump.

Democrats, however, argue it was not Barr’s choice to make and that lawmakers must decide whether the president’s actions rose to the level of “high crimes and misdemeanors.”

The new batch of subpoenas going after Porter, Dearborn and Lewandowski is likely to escalate the standoff between the White House and congressional Democrats, who are conducting a sprawling investigation into possible obstruction outlined in the report and potential abuses of power by the president.

The Trump administration has sought to block the testimony of current and former White House aides including McGahn, who is seen as a key witness in Mueller's report.

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

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

House Democrats are seeking do just that and challenge the White House claims in court.

Earlier this month, they filed a civil lawsuit to enforce a subpoena for testimony from McGahn, arguing that if they win that case, other former and current White House aides would also be compelled to appear on Capitol Hill.