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House impeachment managers and President Donald Trump's defense team faced questions from senators Wednesday as Trump's Senate trial entered a new phase.

The first query, from the three GOP senators who are considered the most likely to vote to continue the trial with additional witnesses, may well have been the most pivotal. Senators remained divided over the issue Wednesday, with Republicans working to get the votes needed to block a call for witnesses.

Here's a look at some of the best — and most important — moments from Wednesday's question-and-answer session.

Dershowitz: "The president is ... irreplaceable"

Responding to a question from Sen. Ted Cruz, R-Texas, about whether quid pro quo arrangements are often used in foreign policy, Trump lawyer Alan Dershowitz suggested that there would have been nothing wrong with the president's seeking foreign help for his re-election.

That's because a president could rightfully think that his re-election is in the country's best interest, he said.

"If a president does something which he believes will help him get elected in the public interest, that cannot be the kind of quid pro quo that results in impeachment," Dershowitz said. "Every public official I know believes that his election is in the public interest."

Schiff said that if the Senate adopted that stance, it would give "carte blanche" for more foreign interference in the future.

Constitutional scholars blasted Dershowitz's argument as "outrageous" and "preposterous."

Erwin Chemerinsky, dean of the University of California, Berkeley, law school, told NBC News he thought Dershowitz's argument was "absurd and outrageous."

"It means that a president could break any law or abuse any power and say that it was for the public interest because the public interest would be served by his or her election," he said.

Sanford Levinson, a University of Texas law professor, said Dershowitz's argument was "on its face, preposterous."

Levinson told NBC News that while candidates for office "make a variety of deals that they would prefer not to in behalf of the good cause" of their election, "we rely on a certain moral compass that will stop at, say, outright bribery" and "suggesting assassinations."

On Thursday, Dershowitz said his comments were being "willfully distorted" by the media.

"They characterized my argument as if I had said that if a president believes that his re-election was in the national interest, he can do anything," Dershowitz tweeted. "I said nothing like that, as anyone who actually heard what I said can attest."

Later Wednesday, Dershowitz argued that the vast majority of legal scholars who disagree with him would feel differently if a Democratic president were on trial.

"What happened is that the current president was impeached," he said. "If, in fact, President Obama or 'President' Hillary Clinton had been impeached, the weight of current scholarship would be clearly in favor of my position.

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"These scholars are influenced by their own bias, by their own politics, and their views should be taken with that in mind," he added. "They simply do not give objective assessments of the constitutional history."

As senators' questions stretched into the evening, Dershowitz argued that the difference between impeaching a judge and impeaching a president is essentially a mathematical one.

"The president is the executive branch. He is irreplaceable," Dershowitz said emphatically in response to a question from five GOP senators who asked how the framers would view removing a president if only one political party supported the president's impeachment.

Collins' question gets at witness issue

Right off the bat, Chief Justice John Roberts read the White House lawyers a question from Sens. Susan Collins of Maine, posed on behalf of herself, Lisa Murkowski of Alaska and Mitt Romney of Utah.

"If President Trump had more than one motive for his alleged conduct, such as the pursuit of personal political advantage, rooting out corruption and the promotion of national interests, how should the Senate consider more than one motive in its assessment of Article One?" the question said, referring to the first article of impeachment alleging abuse of power.

Trump lawyer Patrick Philbin replied that the House managers charged that Trump's motives for withholding aid to Ukraine were purely personal and that if his client had mixed motives for withholding aid from Ukraine, the House's case falls apart.

"Once you're into mixed motive land, it's clear that their case fails. There can't possibly be an impeachable offense at all," he said, adding that it would be impossible to know how much each factor weighed in Trump's mind.

Rep. Adam Schiff, D-Calif., the lead House manager, told the senators that Trump's motive was "clear" and "corrupt." If senators have "any questions about whether it was the factor or a factor," they should call former national security adviser John Bolton to testify about a conversation he had with the president on the subject.

Full coverage of President Donald Trump's impeachment trial

Executive privilege

Republicans asked two questions about why House managers never sought legal challenges to White House claims of executive privilege.

Rep. Hakeem Jeffries, D-N.Y., one of the House managers, gave a simple explanation — the White House never actually claimed executive privilege.

"What the president did raise was this notion of blanket defiance," ordering executive branch employees not to cooperate with or turn over documents to the House impeachment inquiry, Jeffries said.

Philbin offered a different explanation. "The reason there was no attempt is that the House Democrats were just in a hurry," he said.

Why the interest in the Bidens now?

Collins and Murkowski also questioned White House lawyers about whether the president expressed any concerns about former Vice President Joe Biden and his son before Biden declared that he was running for president and, if so, "what did the president say to whom and when?"

Philbin said, "I can't point to something in the record that shows President Trump at an earlier time mentioning specifically something related to Joe or Hunter Biden," but he suggested that the information had recently come to Trump's attention because it had just been collected by his lawyer Rudy Giuliani.

Schiff responded: "Why did he stop the aid this year and not the prior year? Are we to believe it's merely a coincidence it was the year Joe Biden was running for president?"

Bolton's long shadow

House managers used several of their answers to argue that senators should call Bolton to testify in the wake of a bombshell New York Times report that he discussed the frozen aid with the president.

Philbin said that if the Senate were to call witnesses, the trial would "drag on for months."

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Referring to the Senate impeachment trial of President Bill Clinton in 1999, Sen. Tom Carper, D-Del., then asked the House managers: "Isn't it true that depositions of the three witnesses in the Clinton trial were completed in only one day each? And isn't it true that the chief justice as presiding officer in this trial has the authority to resolve any claims of privilege or other witness issues without any delay?"

Jeffries replied, "Mr. Chief Justice, the answer is yes."

Philbin countered that testimony from Bolton would lead to a lengthy fight over executive privilege.

Whistleblower ties

Republicans asked two questions about the whistleblower who complained about Trump's conduct in his call with Ukrainian President Volodymyr Zelenskiy on July 25, including whether the person ever spoke to Schiff or worked with Joe Biden.

Schiff said he doesn't know who the whistleblower is and wants to keep it that way.

"The whistleblower did what they should, except to the president, that's unforgivable," because it exposed Trump's wrongdoing, Schiff said. He said that the allegations in the whistleblower's complaint had been corroborated and that calling the person as a witness would "endanger the whistleblower's life."