Romney, 70, was the Republican Party's presidential standard-bearer in 2012, eventually losing the general election to then-President Barack Obama.
With Romney's potential arrival in Washington comes notably real tension between him and the man now occupying the Oval Office: President Donald Trump.
Romney called Trump a "phony" and a "fraud" during the 2016 presidential campaign, and Trump has regularly taken to Twitter to criticize Romney.
He met with Utahans throughout the state Friday to kick off his campaign, a Romney source told ABC News.
In his first formal public appearance on camera since announcing his campaign, Romney delivered the keynote speech Friday night for the Utah County Republican Party's annual Lincoln Day Dinner in Provo, Utah.
Romney spoke for a little over a half hour in his first on-camera remarks and addressed criticism of his tenuous connection to the state of Utah right off the bat.
"It is an emotional time for me to come back and be in this city," Romney said. "Ann and I lived here for three years while we were going to school. This is where our oldest son was born, at Utah Valley Hospital. I remember it very, very well the feelings as we drove into town, looked to see how much of it remained the same, and how much of it is different."
Romney also addressed gun violence, saying the school shooting Wednesday in Parkland, Florida, was "unthinkable and unimaginable." In a departure from many Republicans in Congress, including Speaker Paul Ryan, he said now was the time to discuss fixes for the problem.
"As a legacy to those who have been killed and lost, we must take action to prevent tragedies like this from ever happening again -- and heaven forbid from happening in our great state," he said. "I believe this is a time for us to have this discussion because it is very much in our minds. I don't know what the answers are to make our schools safer. I don't have all the answers, I've got some ideas. But I think we can't just sit and wait and hope and expect things are gonna get better, because these things just keep on happening.
"Now I've looked at some federal legislation -- I don't see any federal legislation that would have prevented these attacks and so I don't support passing some new federal law of the nature I described. Although an exception may be for Sen. Orrin Hatch's proposal, that he's been promoting, that's been for enhanced background checks. My own view, having served as a governor, is that the best place for finding solutions for school violence is going to be at the state and local level where things are able to happen at a more advanced, responsive manner.
He was light on specifics, but mentioned enhanced security at schools and police patrols, as well as intervention teams for children with mental health issues and "perhaps the age and psychological restrictions on gun purchases."
Romney, never known for his sense of humor, did manage to get a few cracks off, saying he was taking questions from submitted on social media to "break up the monotony of my 3 1/2-hour speech."
"I do want to dispel the rumor that I only ran for president as a stepping stone to become U.S. senator from Utah," Romney joked.
Romney also touted his ability to work across the aisle in his time as a Republican governor in Massachusetts with what he called a "90 percent Democratic legislature."
Republican Utah Gov. Gary Herbert, Sen. Mike Lee, R-Utah, and Reps. Mia Love, R-Utah, and John Curtis, R-Utah, were expected to attend the speech, according to a dinner invitation obtained by ABC News.
The Utah County Republican Party had lined up the former Republican presidential nominee as its keynote speaker before Romney announced his intentions to run for Senate and there was no coordination between the party and campaign, Craig said.
The seat Romney is seeking is held by longtime Sen. Orrin Hatch, R-Utah, who announced earlier this year that he will not seek re-election in 2018.
In his announcement video today, Romney does not mention Trump but did have a few thoughts on the immigration debate waging in Washington.
"Utah welcomes legal immigrants from around the world,” Romney says. “Washington sends immigrants a message of exclusion.”
After taking some time off from politics, Romney also said, he and wife Ann decided that running for the Senate was the right decision.
"Over the last five years, Ann and I have spent a good deal of time with our 24 grandchildren. I've gone back to business, campaigned for Republicans and met with young people across the country," Romney says in the video. "Given all that America faces, we feel that this is the right time for me to serve our state and our country."
Romney's GOP running mate in the 2012 presidential campaign, House Speaker Paul Ryan of Wisconsin, reacted to the news Friday, releasing a statement that read in part: "This is a terrific day for the United States Senate."
"I was honored to join Mitt Romney on the national ticket in 2012 and couldn't be happier that he's running for this seat," Ryan added. "Our party and our country are always better off when Mitt is engaged, and I know that he will put his unparalleled experience, conservative leadership, and life time of service to work for Utah in the U.S. Senate."
While the primary and general election races to succeed Hatch are not expected to be competitive, Democrats still criticized Romney's announcement, saying he "desperately wants to separate himself from the extremism of the current administration."
"Utah deserves a Senator who will expand opportunities and fight to improve the lives of everyone, not another multi-millionaire looking out for himself, his rich neighbors and the special interests," Democratic National Committee spokesman Vedant Patel wrote in a statement released this morning.