A prominent Republican senator, Mitt Romney, broke with his party on Wednesday and announced he would vote to convict Donald Trump in his impeachment trial just hours before the U.S. Senate was poised to acquit the Republican president.

The Republican-controlled Senate was scheduled to hold its historic vote at 4 p.m. EST on whether to convict Trump on charges of abuse of power and obstruction of Congress arising from his dealings with Ukraine in only the third presidential impeachment trial in U.S. history. The Democratic-led House of Representatives approved the charges on Dec. 18.

Romney, the 2012 Republican presidential nominee who has previously criticized Trump on other matters, called the president’s actions in pressuring Ukraine to investigate political rival Joe Biden “grievously wrong” and said Trump was “guilty of an appalling abuse of public trust.”

1:41 Donald Trump’s impeachment trial expected to end with acquittal Donald Trump’s impeachment trial expected to end with acquittal

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“What he did was not ‘perfect,'” Romney said on the Senate floor, as Trump has described his call with Ukraine’s president that was at the heart of the scandal. “No, it was a flagrant assault on our electoral rights, our national security and our fundamental values. Corrupting an election to keep one’s self in office is perhaps the most abusive and destructive violation of one’s oath of office that I can imagine.”

Romney, a moderate and elder statesman in his party, is the only Republican senator to announce that he plans to vote to remove Trump from office.

The impeachment drama that has consumed Washington since September will come to an end with Wednesday’s vote, allowing Trump to pivot toward winning re-election in November.

While the vote is historic, the outcome of acquittal appears assured. A two-thirds majority vote would be necessary to remove him. Republicans hold 53 of the Senate’s 100 seats, and none of them has called for conviction. Trump, America’s 45th president, would have to turn over his office to Vice President Mike Pence if convicted on either charge.

4:39 U.S. President Trump delivers his third State of the Union address U.S. President Trump delivers his third State of the Union address

After facing the darkest chapter of his presidency, Trump, 73, is seeking a second four-year term in the Nov. 3 election.

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As they did on Tuesday, senators ahead of the vote made a series of speeches explaining their decision in the trial. It remains to be seen whether any Democratic senators break ranks from their party to hand Trump a bipartisan acquittal.

Democratic Senator Doug Jones, facing a tough re-election bid this year in Republican-dominated Alabama, said he would vote to convict Trump on both charges and voiced alarm at the arguments by the president’s lawyers in favour of virtually unchecked presidential power.

Jones also issued an appeal for unity at a time of deep partisan divisions in Washington and nationwide.

READ MORE: Trump delivers 2020 State of the Union on the eve of his impeachment verdict

“Our country deserves better than this. They deserve better from the president. They deserve better from the Congress. We must find a way to come together to set aside partisan differences and to focus on what we have in common as Americans,” Jones said.

The impeachment charges against Trump centred on his request that Ukraine investigate political rival Biden and his son Hunter Biden and the president’s subsequent actions to block testimony and documents sought by the House in its impeachment investigation.

Democrats accused Trump of abusing his power by withholding $391 million in security aid passed by Congress to help Ukraine combat Russia-backed separatists as leverage to pressure Kiev to help him smear Biden, who is seeking his party’s nomination to challenge Trump in November.

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Trump denied wrongdoing and most Republicans in the House and Senate rallied around him. Over the past few days, some Republican senators have criticized Trump’s behavior but said it did not warrant his ouster.

3:04 U.S. Senate appears poised to acquit Trump after trial U.S. Senate appears poised to acquit Trump after trial

“I hope our Democratic colleagues will finally accept the results of this trial, just as they have not accepted the results of the 2016 election,” said Republican Senator John Cornyn, saying he hopes they do not launch a second impeachment inquiry. “It’s time for our country to come together, to heal the wounds that divide us.”

‘Two systems of justice’

Democratic Senator Kamala Harris said the trial underscored that “in America there are two systems of justice: one for the powerful and another for everyone else.

“Donald Trump knows all this better than anybody,” Harris said.

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READ MORE: Senate appears split along party lines on Trump impeachment acquittal

“After the Senate votes today, Donald Trump will want the American people to feel cynical. He will want us not to care. He will want us to think he is all powerful and we have no power. We’re not going to let him get away with that,” Harris added.

The House launched its impeachment inquiry in September. The Senate trial began on Jan. 16. Senate Republicans voted down a Democratic bid to call witnesses such as former national security adviser John Bolton and present new evidence in the trial.

House Judiciary Committee Chairman Jerrold Nadler told reporters that Democrats likely would subpoena Bolton, who in an unpublished book manuscript described Trump as playing a central role in pressuring Ukraine, as they continue to investigate the president.

Trump is the third U.S. president to have been impeached. The two others, Bill Clinton in 1999 and Andrew Johnson in 1868, were left in power by the Senate.