Donald Trump moved a step closer to winning perfect Republican support on Tuesday in his quest for impeachment acquittal, gaining a vow from senator Susan Collins of Maine to vote to acquit on both articles of impeachment.

Before a final vote scheduled for Wednesday afternoon on whether to convict Trump, senators began a round of speeches on how they think the third presidential impeachment trial in US history should end.

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As he prepared to deliver the State of the Union address at the Capitol on Tuesday night, Trump appeared headed for acquittal on charges of abuse of power and obstruction of Congress. About half of the 100 senators had delivered speeches by Tuesday evening, with no signs of partisan defections.

But a small sense of suspense continued to attend the trial, over the question of whether Trump and his surrogate, the Senate majority leader, Mitch McConnell, would maintain total party discipline.

Play Video 1:00 Senate leaders make speeches before final vote – video

“We must vote to reject the House abuse of power,” said McConnell. “Vote to protect our institutions. Vote to reject new precedents that would reduce the framers’ design to rubble … vote to acquit the president of these charges.”

The effort got a boost from Collins on Tuesday afternoon. “Impeachment of a president should be reserved for conduct that poses such a serious threat to our governmental institutions as to warrant an extreme step of immediate removal from office,” she said. “I do not believe that the House has met its burden in showing that the president’s conduct, however flawed, warrants the extreme step of removal from office.”

Just one question mark remained: Mitt Romney of Utah. Both Romney and Collins last week broke with the party to vote in favor of witnesses and new documentation at the trial. The effort failed, 51-49.

“The Republicans refused to get the evidence because they were afraid of what it would show,” said the Democratic minority leader, Chuck Schumer, on Tuesday. “And that’s all that needs to be said.”

One Republican who had been closely watched as a possible swing vote, Lisa Murkowski of Alaska, ultimately voted with her caucus on the witnesses question. She said on Monday evening she would do so again on Wednesday.

In a speech on a virtually empty Senate floor, Murkowski called Trump’s conduct “shameful and wrong” but saved her hottest fire for Congress itself.

“The House failed in its responsibilities and the Senate – the Senate should be ashamed by the rank partisanship that has been on display here,” Murkowski said. “So many in this chamber share my sadness for the present state of our institutions. It’s my hope that we’ve finally found bottom here.”

For the duration of the trial, Trump and Republicans have sought to derail impeachment by criticizing the conduct of the process on the House side, as Murkowski did.

But the House managers pointed out that they were guided by Republican rules in investigating the case, and their conduct was historically efficient, lasting four months from initiation to final vote.

One potential Democratic crossover vote, Joe Manchin of West Virginia, offered a bill to censure Trump on Monday, and did not say which way he would vote on conviction. The censure bill did not attract any visible interest from McConnell.

In a reprise of a previous controversy at the trial, Republican Rand Paul of Kentucky appeared on the Senate floor on Tuesday with a placard bearing the text of a question he had attempted to ask last week, only to have the question refused by supreme court Chief Justice John Roberts, who is presiding.

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The question included a name that reports have associated with the whistleblower whose August complaint set the impeachment inquiry in motion. Republicans have sought to take pressure off Trump by attacking the whistleblower as a partisan, despite federal laws protecting whistleblowers from recrimination.

Murkowski’s criticism of Trump’s “shameful” conduct was as sharp as any criticism to be heard of Trump among Republicans on the floor on Monday.

Chuck Grassley of Iowa was more typical of the Republican position, saying he did not see charges against Trump as impeachable because “a president is not prohibited by law from engaging the assistance of a foreign ally in an anti-corruption investigation”.

Trump was impeached for using the power of his office in an attempt to extract political favors, retrospectively dressed up as an anti-corruption investigation, from Ukraine.