The Republican has previously referred to Donald Trump as a 'phony and fraud'
Former US presidential candidate Mitt Romney has announced plans to run for the US Senate.
The Republican - who ran against Barack Obama in 2012 - today unveiled his campaign to contest a soon-to-be-vacant Senate seat for Utah.
There are 33 Senate seats up for grabs in November's mid-term elections, along with all 435 seats in the House of Representatives.
The seat is currently held by veteran politician Orrin Hatch, who is set to retire after more than 40 years.
Mr Romney, meanwhile, was one of the most high-profile Republican critics of Donald Trump during the 2016 presidential campaign, claiming: "Donald Trump is a phony, a fraud. His promises are as worthless as a degree from Trump University."
He was later, however, photographed dining with then president-elect Trump, reportedly as one of those under consideration for the Secretary of State job (which ultimately went to Rex Tillerson).
Mr Romney had long been expected to reveal plans to run for office, but a planned Thursday announcement was postponed in the wake of the Florida school shooting "out of respect for the victims and their families".
In an announcement video, Mr Romney said: "I have decided to run for United States Senate because I believe I can help bring Utah's values and Utah's lessons to Washington.
"Utah is a better model for Washington, than Washington is for Utah," he added.
While the video largely avoids directly addressing policy issues, he does reference the heated immigration debate in the US capital, arguing: "Utah welcomes legal immigrants from around the world - Washington sends immigrants a message of exclusion.
"On Utah's Capitol Hill, people treat one another with respect."
I am running for United States Senate to serve the people of Utah and bring Utah's values to Washington. pic.twitter.com/TDkas6gD2p— Mitt Romney (@MittRomney) February 16, 2018
Romney unsuccessfully ran for a Senate seat in 1994, but rose to political prominence as Governor of Massachusetts.
After an unsuccessful bid for the Republican presidential nomination in 2008 - which ultimately went to John McCain - Mr Romney secured the 2012 nomination, but the election ultimately saw Barack Obama re-elected for a second term.
He is also known for serving as head of the organising team for the 2002 Winter Olympics in Salt Lake City in Utah.
The upcoming mid-terms are likely to see more traditional Republicans such as Romney running alongside more populist, Trump-like candidates.