“It’s what autocrats do.”
Romney said Trump committed an “appalling abuse of public trust” in pressuring Ukrainian President Volodymyr Zelensky to announce an investigation into potential 2020 rival Joe Biden.
“What the president did was wrong, grievously wrong,” he said.
“There is no doubt in my mind that were their names not Biden, the President would never have done what he did … It was a flagrant assault of our electoral rights, our national security and our fundamental values.”
He said voting to remove a Republican president from office was the most difficult decision he had ever faced.
Explaining his decision to break with the leader of his own party Romney, who is Mormon, said: “As a senator juror, I swore an oath before God to exercise impartial justice.
“I am profoundly religious. My faith is at the heart of who I am.”
He will become the first senator in US history to vote to remove from office a president from his own party.
Romney said he would vote to acquit Trump on the second article of impeachment, obstruction of Congress.