(CNN) President Donald Trump's former deputy national security adviser Charles Kupperman defied a congressional subpoena Monday, failing to appear for a closed-door deposition before House impeachment investigators and throwing a new hurdle into Democrats' plans to quickly gather evidence in their inquiry.

But Democrats said that attempts by Kupperman and others not to comply with subpoenas will not delay their impeachment investigation, regardless of whether they decide to go to court to try to enforce the subpoenas.

"We are not willing to allow the White House to engage us in a lengthy game of rope-a-dope in the courts," House Intelligence Chairman Adam Schiff said Monday after Kupperman did not appear.

Schiff said that he was confident the House would ultimately prevail in court, though he did not say whether the House would pursue that route — given that any court process is likely to take weeks, if not months, to resolve.

"We expect that the court will make short shrift of that argument, but nonetheless, we move forward," Schiff said.

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