Republican Senator Mitt Romney has broken ranks to vote to convict US President Donald Trump of one article of impeachment, saying he is guilty of an "appalling abuse of public trust".
Mr Romney was the only Republican voting to convict Mr Trump who was acquitted by the Senate of two articles impeaching the president for abuse of power and obstruction of Congress.
"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," the 2012 Republican presidential candidate said.
"The president is guilty of an appalling abuse of public trust," he said in a somber speech on the Senate floor just hours ahead of the historic vote.
Mr Romney found Mr Trump guilty on just the first charge of abuse of power, voting with his Republican colleagues to clear him of obstruction of Congress.
Mr Romney said it was the "most difficult decision I have ever faced" and he expected he would come in for harsh criticism from fellow Republicans.
"I'm aware that there are people in my party... who will strenuously disapprove of my decision, and in some quarters I will be vehemently denounced," he said.
But he said it was a matter of conscience and in accordance with his Mormon faith.
Mitt Romney was the only Republican to vote to convict Donald Trump. Source: AAP
"As a senator juror, I swore an oath before God to exercise impartial justice," he said.
"With my vote, I will tell my children and their children that I did my duty to the best of my ability," Romney said.
Mr Trump was impeached by the Democratic-controlled House on 18 December for withholding military aid to Ukraine to pressure Kiev to investigate his potential November election rival Joe Biden.
Republicans hold a 53-47 majority in the Senate and a two-thirds majority - or 67 senators - is needed to remove a president from office.
