Donald Trump has promoted a conspiracy theory that Mitt Romney is a “secret asset” of the Democratic Party just hours after the Republican senator voted to convict the US president in his impeachment trial of abuse of power.

Mr Romney, a former Republican presidential nominee, delivered a damning speech to the Senate, accusing Mr Trump of an “appalling abuse of public trust” in attempting to pressure Ukraine to open investigations into a political rival in exchange for military aid.

Despite Mr Romney contributing to the bipartisan vote to convict Mr Trump, every other Republican senator sided with the president, meaning he was acquitted on the abuse of power charge by 52 votes to 48.

Following the acquittal on Wednesday, Mr Trump tweeted a video in which a voiceover condemns “slick, slippery, stealthy” Mr Romney for trying to “infiltrate” the Trump administration by “posing as a Republican”.

“Now his cover’s blown – exposed by news reports as a Democrat secret asset,” the voiceover falsely claims. There is no evidence Mr Romney is a Democrat.

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 footage shows Mr Romney thanking Mr Trump for an endorsement of his successful 2018 Senate campaign in Utah, before going on to show news reports of his 2012 presidential election loss to Barack Obama. It concludes with footage of Mr Trump winning the 2016 presidential election.

Mr Trump later tweeted: “Had failed presidential candidate @MittRomney devoted the same energy and anger to defeating a faltering Barack Obama as he sanctimoniously does to me, he could have won the election.”

Mr Trump previously twice posted a shortened version of the video, which omitted the reference to Mr Romney as a “secret asset”.

The first came after Mr Romney said he was “sickened” by Mr Trump’s conduct outlined in the Mueller report, the second time late last year when the senator expressed concern over a phone call with the Ukrainian president.

Mr Trump’s son, Donald Trump Jr, had earlier called for Mr Romney to be expelled from the Republican Party in a tweet falsely claiming he was switching parties.

“Mitt Romney is forever bitter that he will never be POTUS. He was too weak to beat the Democrats then so he’s joining them now,” he said.

“He’s now officially a member of the resistance & should be expelled from the @GOP.”

During his speech from the Senate floor, Mr Romney at times appeared close to tears as he outlined his reasons for sanctioning the removal of Mr Trump.

He said he would vote to convict the president for the abuse of power charge, but acquit him on an obstruction of Congress charge. In doing so, he became the first senator in US history to vote for the removal of a president of his own party in an impeachment trial.

"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 history and the American people.