Under questioning from Democratic counsel Norm Eisen, he conceded that, as he recently opined in the Wall Street Journal, that impeachment does not require the impeached person commit an actual crime. Instead, Turley argued, without the testimony of people like former national security advisor John Bolton, acting White House chief of staff Mick Mulvaney, and freelance diplomat Rudy Giuliani, impeachment "will proceed on a record composed of a relatively small number of witnesses with largely second-hand knowledge of the position." The House Republicans' official Twitter account seized on this point to argue that Democrats are trying to do too much, too soon, and without enough evidence in tow.

Turley is correct to note that the testimony of the aforementioned individuals would strengthen the case for impeachment—or, at the very least, clarify the scope of what Trump did. And Turley's objections to the speed of the inquiry might be compelling if House Democrats were purposefully excluding Mulvaney and Bolton and company from the process.

But throughout the impeachment inquiry, Trump has worked to prevent these individuals and other members of his inner circle from sharing what they know with lawmakers. Giuliani and vice president Mike Pence defied congressional subpoenas for key documents. So has the Office of Management and Budget, an agency led by Mulvaney. Before the public impeachment hearings even began, White House counsel Pat Cipollone sent a letter to Democratic lawmakers vowing that the administration would not cooperate with any aspect of the inquiry, which they decried as "baseless," "unprecedented," and "unconstitutional." Lawsuits seeking to compel appearances of members of Trump's inner circle could take months for federal courts to resolve. Bolton, in particular, has vowed not to testify unless a judge orders it.

At one point, Florida congressman and reliably shouty Trump defender Matt Gaetz asked the panelists to raise a hand if they had "personal knowledge of a single material fact" contained in Intelligence Committee chair Adam Schiff's final impeachment report. None of the law professors raised their hand, of course—a stunt Gaetz intended to undermine their credibility, but that mostly just reiterated the glaring absence of witnesses who could testify about their personal knowledge, if they were willing to do so. The fact is that if Democrats were to wait for the legal process to run its course, their impeachment powers would be, in the meantime, functionally useless. With the 2020 election fast approaching, Democrats have decided to move forward with what they've got.

Besides, as Gerhardt noted, obstruction of Congress is an impeachable act in its own right, and defying lawful orders makes impeachment more appropriate, not less. "There has never been anything like the president's refusal to comply with subpoenas from this body," he told lawmakers. "[The subpoenas] have the force of law to them, and this is something that every president has complied with"—except, he added, for President Nixon, who is probably not someone to whom Trump wants to be compared right now. "The full-scale obstruction of those subpoenas, I think, torpedoes separation of powers, and therefore, your only recourse is to, in a sense, protect your institutional prerogatives," Gerhardt said. "And that would include impeachment." If there is any insufficiency in the evidence, in other words, blame lies more with the administration actively withholding it than it does with anyone else.