After nearly 24 hours, divided across three long days, the prosecution rested their case in the impeachment trial of Donald Trump. There was passion, drama and even a display of “histrionics”, but little suspense.

Mitch McConnell, the Senate majority leader, had revealed the ending weeks ago. Aquittal was inevitable.

The measure of success for the seven impeachment managers appointed by Nancy Pelosi to prosecute the House’s case against the president was therefore not whether Trump was removed from office, but whether they had managed to persuade the Republican-controlled Senate to allow new testimony before a verdict is rendered, perhaps as early as next week.

“Soon members of this body will face the most momentous of decisions, not as I said at the outset of guilt or innocence, but a far more foundational issue,” the congressman Adam Schiff, an eloquent former federal prosecutor and lead impeachment manager, said in an impassioned closing statement. “Should there be a fair trial?”

Of the 100 senators, who sit as judge and jury in the trial, most have already tipped their hand as to how they will vote on the question of whether Trump should be convicted for “high crimes and misdemeanors”. The American public remains deeply divided over whether the president should be removed from office. But on the matter of witnesses, there is the edge of unpredictability. A majority of Americans support the effort and perhaps just enough – four – Republicans are open to it.

In Schiff’s soaring – and lengthy – finish, he seemed to speak directly to these senators, appealing to their sense of American ideals and pleading with them to pursue “moral courage”. He acknowledged the political stakes for senators whose “heartfelt views of right or wrong are in conflict with the popular opinion of our constituents”.

“Those are the times that try our souls,” he said. But then he continued, exaggerating his point. He referenced a CBS story, that quoted a Trump confidant who warned Republican senators that “a vote against the president and your head will be on a pike”.

The mood changed sharply.

Groans of displeasure and disbelief rumbled from the Republican side of the chamber. Senator Susan Collins, a Republican from Maine who has said she is supportive of calling additional witnesses, shook her head, visibly upset.

“That’s not true,” she said, so outraged that she violated the rule that senators remain silent “on pain of imprisonment” during the trial.

Capitol police secure the area around the Senate chamber on the fourth day of the Senate impeachment trial of Donald Trump. Photograph: Erin Scott/Reuters

“He was doing fine with ‘moral courage’ until he got to head on a pike,” Senator Lisa Murkowski, an independent Republican voice in the Senate, told reporters as she left the Capitol. “That’s where he lost me.”

Without their support, Democrats have little chance of cobbling together a coalition to subpoena witnesses and documents in the trial.

But the hour was late and the senators left exhausted. It had been more than nine hours since the Senate chaplain, Barry Black, convened the session with a prayer that the senators maintiain the “wisdom to distinguish between facts and opinions without lambasting the messenger.”

For three days, the impeachment managers delivered what many Democrats – and a few generous Republicans – hailed as a tour-de-force. The team described in elaborate detail a “corrupt scheme” to pressure Ukraine to open an investigation into his political rivals. They used the president’s words against him, in video clips that played from screens installed on the Senate floor. He sought to enrich his political fortunes at the expense of the national interest, and he was bound to do it again, they said.

“President Trump tried to cheat, he got caught and then he worked hard to cover it up,” congressman Hakeem Jeffries, one of the impeachment managers, said, summarizing the charges for abuse of power and obstruction of Congress.

Judiciary Committee Chairman Jerry Nadler, another member of the team, said that Trump’s conduct “puts even president Nixon to shame”. Trump, he said, seeks to be “all powerful.”

“He is a dictator,” Nadler declared.

In his final plea, Schiff warned that the president remains an “imminent threat” and will continue to abuse his power unless Congress acts to remove him from office.

“He is who he is,” Schiff exhorted. “You cannot leave a man like that in office.”

Yet few Republicans were persuaded – or even listening. Instead hey did their best to signal – silently from the chamber and loudly during the brief recesses – that they were bored.

“We saw that protester yell something bad about Chuck Schumer,” Senator Rick Scott, a Republican from Florida, told Fox News. “I’m thinking about possibly doing the same thing so the sergeant-at-arms will take me out also. This is ridiculous. We’ve heard the same thing three days in a row.”

Members darted into the antechamber despite rules requiring them to remain in their seats. Some Republicans openly played with fidget spinners – toys for children who have difficulty focusing. And Senator Pat Toomey, a Republican of Pennsylvania, doled out candy, stashed in a not-so-secret drawer in his desk. On Friday, he announced that Hershey’s, his homestate chocolate manufacturer, had sent reinforcements after stock ran low.

The charge of monotony is exasperating for the House managers, who have argued at great length that there is additional, relevant information just beyond their grasp that the Senate could summon with a simple majority vote.

The American people do not agree on much, but they will not forgive being deprived of the truth Adam Schiff

Trump delighted in their struggle, taunting the Senate minority leader, Chuck Schumer.

Before the proceedings began on Friday, he appendaged his commentary to a tweet by conservative Wall Street Journalist columnist: “Schumer had one job this week: to convince a handful of Rs to join his call for more testimony. He’s done the opposite.”

Democrats won’t know whether there is enough support until next week – after the president’s team delivers its defense, which begins tomorrow. They will have the same 24 hours in a three-day period to make their case before the senators have an opportunity to ask questions.

At that point Schumer has vowed to compel votes on witnesses. And even if they can assemble enough votes to call witnesses, Republicans intend to make it as painful as possible for Democrats and those in their party who defected. They’ve suggested something of a “witness swap” – John Bolton, the president’s former national security adviser who has said he would comply with a subpoena to testify, for Joe Biden, the former vice-president over his son Hunter’s involvement with Burisma, a Ukrainian energy company that had been under investigation.

On Thursday the managers took a calculated risk by rebutting Tump’s baseless assertion Joe Biden sought to remove the Ukrainian prosecutor who was investigating Burisma, a move the president’s allies said “opened the door” to calling Biden as a witness. Democrats are largely opposed to the idea, which they see as a distraction.

Yet if one gratuitous remark in an otherwise sterling speech is enough to topple the crusade for new evidence, then it was unlikely there was ever a chance it could be won in the first place.

But Schiff continued, undeterred.

“The American people do not agree on much, but they will not forgive being deprived of the truth.” Schiff said in closing. “I ask you, I implore you. Give America a fair trial. Give America a fair trial. She’s worth it.”