Senator Mitt Romney says he will vote to convict Donald Trump on one of the impeachment charges he faces, marking a break in the Republican Party ranks just hours before the vote is to be held.

Mr Romney announced his intentions during a speech on the Senate floor, where he said he would vote to convict the president for the abuse of power charge he faces, but acquit him on the obstruction of Congress charge. In doing so, he will be the first senator in US history to vote for the removal of a president of his own party in an impeachment trial.

It appears unlikely that the announcement would significantly alter the likelihood that Mr Trump will ultimately be acquitted on both charges. But, it does denies the president and his allies the ability to cast the effort against him as a solely partisan exercise.

"The president is guilty of an appalling abuse of public trust," Mr Romney said during the speech, in which he invoked his sense of duty to argue that the Senate vote will be in-turn judged by the American people and history books. At times, he appeared to hold back tears when mentioning his being "profoundly religious" and that he had sworn an "oath before God".

"What he did was not perfect," he continued, referencing the president's claims that his actions were flawless. "No, it was a flagrant assault on our electoral rights, our national security, and our fundamental values. Corrupting an election to keep oneself in office is perhaps the most abusive and destructive violation of one's oath of office that I can imagine."

All the president's lawyers: The team fighting Trump's impeachment Show all 6 1 /6 All the president's lawyers: The team fighting Trump's impeachment All the president's lawyers: The team fighting Trump's impeachment Alan Dershowitz Dershowitz is a controversial American lawyer best known for the high-profile clients he has successfully defended. Those clients have included OJ Simpson, Jeffrey Epstein and Harvey Weinstein. One longtime Harvard Law associated told the New Yorker Dershowitz "revels in taking positions that ultimately are not just controversial but pretty close to indefensible." Getty All the president's lawyers: The team fighting Trump's impeachment Ken Starr Starr became a household name in the 1990s as the independent counsel who led the investigation that led to Bill Clinton's impeachment. That investigation began as a look into a real estate scandal known as Whitewater, and eventually led to impeachment after Mr Clinton lied under oath about having an affair with White House intern Monica Lewinsky. AP All the president's lawyers: The team fighting Trump's impeachment Jay Sekulow Sekulow is the president's longtime personal attorney, and, now, personal lawyer in the White House. He has been accused by former Rudy Giuliani associate Lev Parnas of being "in the loop" during the Ukraine scandal. Getty All the president's lawyers: The team fighting Trump's impeachment Pam Bondi Bondi is the former attorney general in Florida, and a longtime backer of the president's. She made a name for herself in Florida for taking hyper partisan stances on issues, and her penchant for publicity. She is likely to be a prominent public-facing figure during the trial. AFP/Getty All the president's lawyers: The team fighting Trump's impeachment Pat Cipollone Cipollone is the White House counsel, and leading the president's defence team. Getty All the president's lawyers: The team fighting Trump's impeachment Rudy Giuliani While not officially named as one of the president's impeachment lawyers, it is hard to ignore Giuliani's outsized role in this process. The former mayor of New York has been making headlines for months as he defends his client, and for his apparent role in the effort to compel Ukraine to launch the investigation into Joe Biden. We'll see how he figures in the actual trial, which he has said he would like to be a part of. Reuters

The Senate will vote on Wednesday afternoon on the articles of impeachment that were delivered from the House following their vote to make Mr Trump the third president in US history to be impeached.

Shortly after Mr Romney's announcement, the White House abruptly cancelled a planned open press meeting in the Oval Office between the president and Venezuelan opposition leader Juan Guaido. The development was billed as a change of plans, and reporters have said it appeared organised.

But, that abrupt cancellation was still met with speculation that the two decisions may be connected, and that the president was not interested in speaking to the press shortly after the surprise announcement. Officially, the White House did not immediately comment on Mr Romney's announcement, and Mr Trump's Twitter account remained silent.

Mr Romney was among a handful of senators who had been watched to see how they would vote, and is the only Republican so far to indicate he would vote to convict.

That group of watched senators includes Democrats as well as Republicans, virtually all of whom represent a state considered to be competitive. Alabama senator Doug Jones, who faces a long-shot re-election bid, said earlier on Wednesday that he would also vote to convict the president.

Susan Collins, a key swing state senator from Maine, has meanwhile announced she will vote to acquit the president.

But Mr Romney's decision also highlights the dangers of breaking from one's party during this impeachment trial, with immediate rebuke to him online with political observers calling for his dismissal from the Republican Party. Months before Mr Romney, congressman Justin Amash similarly found himself under fire for rebuking the president, and ultimately announced he would leave the Republican Party last year.

"Mitt should be expelled from the @SenateGOP conference," wrote Donald Trump Jr, the president's adult son, on Twitter after the floor speech.