The opposing legal teams in Donald Trump’s impeachment trial locked horns directly for the first time on Wednesday, as an interrogation period began in which senators submitted questions in writing to be read aloud by Chief Justice John Roberts, who is presiding.

The format set up clashes over Trump’s motivations, the power of Congress to call witnesses from the executive branch and, most of all, the need – or not – for the former national security adviser John Bolton and possibly other witnesses to testify at the trial.

In its eighth full day, the trial drew ever nearer to a crossroads, expected on Friday, at which the senators will decide whether to call witnesses – or to move on to a vote for Trump’s acquittal.

If the Senate votes to call witnesses, Trump’s legal team warned, “that changes the nature and scope of the proceedings” and could lead to court challenges that would draw the trial out.

Adam Schiff, the lead impeachment manager, argued that Roberts, “a perfectly good chief justice”, could make fast rulings that would prevent the testimony of Bolton or others from creating a lengthy detour in the trial.

“They could no longer contest the facts,” Schiff said of Trump’s defense team. “So now they have fallen back on, ‘You shouldn’t hear any further evidence on this subject.’ Think about the precedent you would be setting if you don’t allow witnesses in a trial.”

The relatively fast-paced question period, which allowed five minutes per response to each of 54 questions before the dinner break, came after a week in which the two sides made strictly siloed opening arguments, speaking for multiple days each to lay out their cases to the senators.

Apart from the legal clashes, the question-and-answer format requiring Roberts to read the senators’ queries produced some awkwardness, as when Kamala Harris, a California Democrat, said that Trump appeared to believe that being president gave him unlimited powers, including the ability to stonewall Congress.

“Nixon said ‘when the president does it it’s not illegal’,” Roberts said, reading Harris’s question. “President Trump said, ‘When you’re a star, they let you do it, you can do anything.’” The words in the chief justice’s mouth were from the Access Hollywood tape that surfaced a month before Trump was elected.

Trump was impeached for abuse of power for allegedly conditioning military aid for Ukraine on the receipt of personal political favors, and for obstruction of Congress for attempting to cover the scheme up.

Trump’s defense team has argued that the president withheld aid from Ukraine out of a desire to combat internal corruption in the country, although prosecutors in the case have pointed out that nowhere else in his life or presidency has Trump taken up anti-corruption measures as a priority.

Any senators with lingering questions about Trump’s motives for suspending aid to Ukraine have an easy solution at hand, Schiff said: call Bolton to testify.

In a manuscript to be published in March, Bolton reportedly describes a conversation with Trump in August 2019 in which Trump said he did not want to release aid for Ukraine until the country announced investigations, including one into the former vice-president Joe Biden.

The White House deputy counsel Patrick Philbin answers a question during the trial. Photograph: AP

“Don’t wait for the book,” Schiff urged the Senate. “Don’t wait for March 17 when it is in black and white. Find out the answer to your question.”

Patrick Philbin, a Trump lawyer, warned that as a former national security adviser, Bolton’s testimony could be protected by “an absolute privilege of confidentiality”.

“To suggest that the national security adviser, we’ll just subpoena him, he’ll come and it’ll be no problem – that’s not the way it would work because there’s a vital constitutional privilege there,” Philbin said. “There would be grave issues raised attempting to have a national security adviser to come.”

But there were few signs that the tug-of-war over Bolton’s testimony would come to a clash of constitutional principles. Democrats still seemed to lack the firm support of the three Republican senators they would need to advance the witnesses question. On Wednesday, one moderate Republican, Cory Gardner of Colorado, announced he would vote against witnesses.

The three Republicans under the most intense scrutiny on both sides of the aisle – Susan Collins of Maine, Mitt Romney of Utah and Lisa Murkowski of Alaska – posed the first question of the session, asking whether Trump’s conduct toward Ukraine would be impeachable if he was acting out of both public and personal motives.

“If there is something that shows a possible public interest,” argued Philbin, “that destroys their case. Once you’re into mixed motive land, their case fails.”

Invited by minority leader Chuck Schumer to reply, Schiff pounced, arguing that “if any part of the president’s motive was a corrupt motive, that is enough to convict”.

“There is a witness a subpoena away who can answer that question,” Schiff said, in one of many calls to subpoena Bolton’s testimony.

Republicans control the Senate with a 53-seat majority. A two-thirds majority of voting senators would be required to remove Trump from office.

As the evening wore on, the two sides fell into a pattern of directing questions exclusively to their own sides – Democrats to the House managers, Republicans to Trump’s legal team.

“It’s highly unlikely that you’ll see either side asking a question of the other side,” the Republican Tim Scott of South Carolina explained during a break. “You don’t want to see someone just drone on.”