He was tapped to be Bolton's right-hand man in January.
Samantha Vinograd is a CNN national security analyst.
But Bolton's move to add another like-minded GOP hawk to Trump's national security team raised immediate questions as Trump had already begun indulging his non-interventionist instincts in the Middle East with his plans to pull out of Syria and withdraw some troops from Afghanistan - in spite of Bolton and other aides' opposition.
"We still have further depositions to do, and I don't want to be committed to a particular timetable, but we are moving with all expedition", Schiff told reporters. Bolton claims he resigned. Pelosi, meanwhile, suggested that this week's votes are necessary to secure the Trump administration's cooperation.
Some Democrats had hoped Kupperman might still appear for his scheduled hearing on Monday, joining other witnesses who have defied White House orders.
Kupperman is one of several less-known US officials summoned to testify this week, while Democrats keep their eye on Bolton, a more prominent player.
Kupperman issued a statement citing the importance of determining the division of powers between the executive branch and Congress.
The White House instructed Kupperman not to appear, CBS reported.
Schiff noted that a federal judge has validated the legality of the Democratic-led impeachment inquiry in a separate decision about releasing information regarding a separate investigation, into Russian election interference in 2016, to Congress and that all witnesses need to come forward when called.
Charles Kupperman failed to show up for a scheduled deposition Monday after asking a federal court in Washington for guidance on whether he was legally required to do so. Democrats consider both to be key witnesses in the impeachment inquiry. Trump used that argument in a lengthy letter to the House earlier this month saying that he wouldn't cooperate.
The three presidents of the lower house committees overseeing the investigation had told Kupperman's lawyers in a previous letter that the lawsuit filed on Friday was unfounded and appeared to be coordinated with the White House.
Respondents were also asked about the House leadership's failure to hold a full House vote on the impeachment inquiry.
In a letter sent Monday to the lawyer of the legislators who conduct the investigation process in the Capitol, the lawyer of Kupperman said his client does not challenge the constitutional claim of the members of Congress to summon him to appear. View more opinion articles on CNN. They still have time to depose Kupperman after courts "make short shrift" of the administration's argument.
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