The head of the House impeachment inquiry said Monday that the failure of four White House officials to testify despite subpoenas adds to the evidence against President Donald Trump.

The four who defied the subpoenas are John Eisenberg, legal adviser to the National Security Council, his deputy, Michael Ellis, as well as Robert Blair, a top aide to acting White House chief of staff Mick Mulvaney, and Brian McCormack, an aide at the White House Office of Management and Budget who previously worked for Energy Secretary Rick Perry.

The refusals to escalated the legal battle between House Democrats and the Trump White House, which has vowed to stonewall the inquiry into whether the president used nearly $400 million of U.S. military aid as leverage in an attempt to force Ukraine's government to investigate the son of Trump's political rival, former Vice President Joe Biden.

House Intelligence Committee Chairman Rep. Adam Schiff, D-Calif., told reporters the failure to appear despite subpoenas would be considered by Democrats as "further evidence of an effort by the administration to obstruct the lawful and constitutional duties of Congress."

"This will only add to the body of evidence on a potential obstruction of Congress charge against the president," Schiff said, noting that during the impeachment inquiry into former President Richard Nixon, an article of impeachment was based on obstruction of Congress.

"Today, we have four additional subpoenas to add to the list of a potential charge involving the president of the United States and his obstruction of our constitutional duties," Schiff said.

In the case of Eisenberg, the White House informed his attorneys that the president would block his testimony by invoking a sweeping form of executive privilege known as "constitutional immunity," according to a letter Sunday from White House Counsel Pat Cipollone to Eisenberg's lawyer.

Eisenberg's testimony is considered especially relevant to the inquiry because of his central role in the immediate aftermath of the July 25 phone call in which Trump asked Ukrainian President Volodymyr Zelensky to investigate Hunter Biden and a Ukrainian company where he served on the board.

In testimony last week, the National Security Council's top expert on Ukraine, Lt. Col. Alexander Vindman, testified that Eisenberg took the unusual step of seeking to transfer a reconstructed transcript of the phone call to a highly classified computer server, as opposed to a more widely accessible server on which the transcripts of calls to foreign leaders are typically stored.

House investigators are hoping Ellis can shed light on some of the same events in the aftermath of the July call.

Trump has denied any wrongdoing in his dealings with Ukraine and repeatedly attacked the impeachment process. On Monday, he wrote on Twitter, "What I said on the phone call with the Ukrainian President is "perfectly" stated. There is no reason to call witnesses to analyze my words and meaning. This is just another Democrat Hoax that I have had to live with from the day I got elected (and before!)."

Blair, who is one of the few people who actually listened to the call, is believed to have specific information about a halt to U.S. military aid earmarked for Ukraine, which was ordered by Trump and conveyed through Mulvaney.

Over the weekend, Blair's lawyer told CNN, "Mr. Blair is caught between the assertions of legal duty by two coequal branches of government, a conflict which he cannot resolve." NBC News reported that Blair received a subpoena on Sunday.

The fourth witness, McCormack, could potentially offer House impeachment inquiry investigators a fresh perspective into several key events leading up to the phone call, including any collaboration between Perry and Trump's personal lawyer, Rudy Giuliani, during the spring and summer.

According to testimony from several current and former administration officials in recent weeks, Perry was one of three administration officials who were charged with running a shadow foreign policy toward Ukraine and working closely with Giuliani.

McCormack received a subpoena on Sunday, according to NBC News. Perry is scheduled for a deposition on Wednesday, but the Department of Energy has already indicated that the secretary will not appear.

The House investigation focuses on whether Trump abused the power of his office in his attempt to pressure Ukraine to investigate a political rival, and if so, whether those actions meet the standard for "high crimes and misdemeanors" deserving of impeachment and, potentially, removal from office.