There does not appear to be four Republican senators who will vote to allow witnesses, a break from every other impeachment trial in American history.
An end to Donald Trump's impeachment trial appears closer after one influential Republican senator said he would not support a call for more witness.
Earlier, one of Trump's attorneys tried to explain his controversial defense. Meanwhile, another Republican senator was dramatically thwarted in his questioning by Chief Justice John Roberts.
Highlights of Thursday's session and what's ahead as senators conduct just the third impeachment trial of a president.
CAN I GET A WITNESS?
Tennessee Republican Sen. Lamar Alexander's announcement late Thursday that he will oppose calling more witnesses all but dashed Democratic efforts to hear more testimony.
At the same time, another key Republican, Sen. Susan Collins of Maine, said she would vote for more witnesses, but it would take four GOP senators to break with the majority and join with Democrats to tip the outcome.
"There is no need for more evidence," Alexander said in his statement.
While he said it was "inappropriate" for Trump to ask Ukraine to investigate a political rival, he said the House charges, even if true, "do not meet the Constitution's 'treason, bribery, or other high crimes and misdemeanors' standard for an impeachable offense."
Collins said she wanted the House Democrats prosecuting the case and Trump's defense attorneys to agree on "a limited and equal number of witnesses for each side."
A vote on witnesses, expected Friday, could lead to an abrupt end of the trial with the expected acquittal.
CLEAN-UP ON AISLE FIVE
Alan Dershowitz, one of Trump's lawyers and a renowned Harvard professor, delivered a stunning defense of Trump on Wednesday night that essentially would make it impossible to impeach a president for anything he might do to boost his reelection prospects. The argument was quickly and forcefully denounced by legal scholars, historians, and editorial boards who said there are clear limits on presidential authority.
Dershowitz seemed to backtrack Thursday, saying his remarks have been misinterpreted. But Democrats seized on them as they pressed their case for Trump's removal from office for tying the release of military aid to Ukraine to an investigation of his political rivals.
"I never said a president could do anything to get re-elected," Dershowitz said on Twitter. "A president seeking re-election cannot do anything he wants. He is not above the law."
But Democrats said that is exactly what Dershowitz claimed on Wednesday: "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," he said on the Senate floor.
Rep. Adam Schiff, the lead Democratic prosecutor called Dershowitz's argument "a descent into constitutional madness."
Republican Sen. Rand Paul of Kentucky roiled the trial briefly Thursday as he tried to ask about the intelligence community whistleblower whose complaint led to the investigation that resulted in impeachment. Paul has said he believes the whistleblower may have conspired with House staff aides in writing the August complaint.
Roberts, who has told senators the whistleblower's identity should be protected, refused to read the question aloud. He did not say what was on the card.
Paul left the impeachment proceeding to hold a news conference where he claimed his question was not aimed at outing the whistleblower.
"I don't know who the whistleblower is. ... My question's not about the whistleblower," Paul said. "My question's about two people who were friends who worked together at the National Security Council who have been overheard talking about impeaching the president years in advance."
U.S. law protects the identity and careers of people who bring forward accusations of wrongdoing by government officials. Lawmakers in both parties have historically backed those protections.
In a rare show of bipartisanship, two Democrats — Arizona Sen. Kyrsten Sinema and West Virginia's Joe Manchin — joined with two Republicans, Alaskan Lisa Murkowski and Susan Collins of Maine to ask a question.
The lawmakers wanted to be assured by Trump that private citizens would not conduct foreign policy unless they've been specifically designated by the State Department to do so. The question referred to Trump's personal attorney, Rudy Giuliani, who played a key role in the Ukraine investigation.
White House lawyer Patrick Philbin said Giuliani was merely a "source of information" for Trump on Ukraine and was not conducting foreign policy. Even George Washington relied on trusted confidants, Philbin noted.
Schiff called the answer "breathtaking," saying the White House was admitting that Giuliani was not conducting official business when he was in Ukraine or working on Ukraine issues.
"They have just undermined their entire argument," Schiff said, adding that the White House had "acknowledged that the person in charge of this was not conducting policy" but instead "was undergoing this domestic errand."
The last comment referred to a remark by former Trump adviser Fiona Hill, who testified that Giuliani and his associates were on "a domestic political errand" while she and other professionals were engaged in foreign policy.
ECHOES OF WATERGATE
Rep. Zoe Lofgren of California, a congressional staffer during Watergate and now a House prosecutor, said Watergate also started as a partisan inquiry. A bipartisan consensus emerged only after Republicans — including staunch Nixon supporters — saw enough evidence to change their minds, she said.
"They couldn't turn away from the evidence that their president had committed abuse of power and they had to vote to impeach him," Lofgren said. While disappointed that House Republicans did not vote to impeach Trump, the Senate — "the greatest deliberative body on the planet" — has a new opportunity, she said.
Unlike House investigators, the Senate can call first-hand witnesses like Bolton who can testify about what they heard Trump say and saw him do, Lofgren said. "I think we would all learn something. Let's see if that kind of information can help the senators come together as happened in the House Judiciary Committee so many years ago."
A vote on witnesses, expected Friday, could lead to an abrupt end of the trial with an expected acquittal by Republican senators unconvinced of wrongdoing by Trump. A less likely but still possible outcome was a deal that could bring testimony from Bolton and other witnesses.
Two GOP senators — Collins and Utah's Mitt Romney — are in favor of calling witnesses. If a third Republican joins them, that could lead to a 50-50 tie on the question. It was not clear if Roberts would attempt to break a tie.