Romney announced his campaign in a video message. Already fighting criticism that he is an outsider, the 2012 Republican presidential nominee mentioned Utah early and often in the clip.
"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," he said. "Utah is a better model for Washington than Washington is for Utah."
Romney will aim to replace 83-year-old Sen. Orrin Hatch in November's election. The longtime senator announced his retirement in January even as Trump pushed him to run again.
Romney, 70, has a a strong chance to win the seat later this year. While not a consistent Utah resident in years past, he has strong name recognition and is considered popular in the state. Romney is a Mormon who helped to reorganize the scandal-plagued 2002 Olympics Games in Salt Lake City — an effort he highlighted in his announcement.
Romney's run has already faced some resistance: the head of the state's Republican Party criticized him for not having deep enough ties to the state. Jenny Wilson, a Democratic candidate running for Senate in Utah, said this week that "Utah families deserve another Utahn as their senator, not a Massachusetts governor who thinks of our state as his vacation home."
If elected, the former governor would bring strong name recognition and influence as a first-term senator. While former aides expect Romney to push for conservative policies in the Senate, they also believe he will rebuke the president when necessary and potentially clash with him on some policies.
Romney likely would have backed the Republican tax law passed in December. But he may break with Trump on topics such as relations with Russia and immigration.
In his announcement Friday, the former governor took a swipe at immigration hardliners in the White House and Congress.
"Utah welcomes legal immigrants from around the world. Washington sends immigrants a message of exclusion," he said.
In the video, Romney also criticized the level of national debt and the lack of civility in Washington. He promoted an export-driven economy.
Romney heavily criticized then-candidate Trump in a 2016 speech, calling him a "phony" and a "fraud." He warned that Trump would cause economic instability and endanger Americans abroad.
Later, Romney unsuccessfully interviewed to be Trump's secretary of State. Since, he has publicly rebuked Trump when he supported Roy Moore, the Alabama Senate candidate accused of sexually abusing teenagers, and when the president reportedly questioned why the U.S. needed immigrants from "s---hole" African countries.
If he wins the seat and criticizes Trump while in office, he would mark a stark shift from Hatch. Hatch has heaped praise on the president in recent months, calling him a "heck of a leader" after the GOP passed its tax plan in December. Trump reportedly begged the 83-year old Hatch to run for re-election one more time.
Before entering politics, Romney led investment firm Bain Capital, a spin-off of Bain & Company. After his role in the Utah Olympics, he served as Massachusetts governor from 2003 to 2007.
There, he oversaw the creation of a health insurance program that some consider a precursor to the Affordable Care Act. As a presidential candidate in 2012, he pledged to repeal Obamacare and pushed for marketplaces created by states.
Romney unsuccessfully ran for president in 2008 before winning the GOP nomination in 2012, when he lost in the general election to incumbent President Barack Obama.