Homeland security adviser Tom Bossert is slamming Senate Armed Services Committee Chairman John McCain John Sidney McCainBiden's six best bets in 2016 Trump states Replacing Justice Ginsburg could depend on Arizona's next senator The Hill's Morning Report - Sponsored by Facebook - Washington on edge amid SCOTUS vacancy MORE (R-Ariz.) for drawing attention to a chair left empty when cybersecurity czar Rob Joyce did not testify at a panel hearing on Thursday

"I thought that empty chair stunt was cheap and beneath him," Bossert said Tuesday.

The Trump administration reportedly withheld Joyce from testifying at the hearing on defending against cyberattacks due to his position at the White House not being confirmed. Unconfirmed advisers traditionally do not testify, despite Joyce holding a unique place in cybersecurity policy decisions.

“I would also like to note at the outset the empty chair at the witness table,” McCain said during his opening remarks. “Unfortunately, but not surprisingly, the White House declined to have its cyber coordinator testify.”

McCain has been a frequent critic of President Trump, with the commander in chief going so far as to warn the Arizona Republican last week, "At some point I fight back, and it won't be pretty."

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Bossert struck back at McCain talking with reporters after leaving the stage at the Federal Ignite conference held by Palo Alto Networks in Washington, D.C.

During his appearance at the conference, Bossert addressed the lack of a unified "vision" for improving cybersecurity.

Bossert argued that discussions throughout the conference, including by Rep. John Ratcliffe (R-Texas), about a coordinated public and private effort to bolster global cybersecurity would not work unless all parties agreed on the same goal. He added that the process of deciding that goal could take years, let alone the time it would take to reach that goal.

"If we come out with a unifying vision [during the Trump administration], I'd be thrilled. If we come up with a way to do it in four years, I'd be astonished," he said.