Mitt Romney and President Donald Trump have harshly criticized each other but also have been willing to sit down to talk when it's mutually beneficial.
Romney's announcement Friday that he is running for the U.S. Senate in Utah creates the potential for more battles or deal-making.
A look back at some of the most memorable moments in recent years between the two wealthy businessman and politicians:
2011: TRUMP DERIDES ROMNEY AS 'SMALL BUSINESS GUY'
In an interview with CNN, Trump dismissed Romney as a "small business guy" and suggested Bain Capital, the venture capital firm where Romney made his millions, had bankrupted companies and destroyed jobs.
Trump said that he had a "much, much bigger net worth. I mean my net worth is many, many, many times Mitt Romney."
Later that year, Romney turned down an invitation to participate in a presidential debate that Trump planned to moderate in Iowa in December, leading Trump to cancel the event.
2012: TRUMP ENDORSES ROMNEY FOR PRESIDENTIAL BID
Trump endorsed Romney in the 2012 GOP presidential primary inside a hotel complex that bears Trump's name on Las Vegas Strip.
"There are some things you just can't imagine happening. This is one of them," Romney said with a smile on Feb. 2, 2012. He praised Trump's ability to "understand how our economy works and to create jobs for the American people."
Trump said Romney was "tough, he's smart, he's sharp and he's not going to allow bad things to continue to happen to this country we all love."
In 2016, amid a war of words with Romney, Trump accused Romney of "begging me" for the endorsement. "I could have said, 'Mitt, drop to your knees.' He would have dropped to his knees," Trump said.
MARCH 2016: ROMNEY CALLS TRUMP 'A PHONY'
In a scathing speech delivered at the University of Utah during the 2016 GOP presidential primary, Romney blasted Trump as "a phony" and "con man" whose "imagination must not be married to real power."
"His is not the temperament of a stable, thoughtful leader," Romney declared. "If we Republicans choose Donald Trump as our nominee, the prospects for a safe and prosperous future are greatly diminished."
Trump called Romney "a failed candidate." He said Romney's candidacy in 2012 "was an embarrassment to everybody" including the Republican Party.
JUNE 2016: TRUMP CALLS ROMNEY 'A CHOKE ARTIST'
Campaigning in a pair of crucial battleground states, Trump called Romney a "sad case."
"You know what a choke artist is?" Trump said. "You know a guy who missed a kick, you get rid of him, right? He choked like a dog."
OCTOBER 2016: ROMNEY CRITIQUES TRUMP'S COMMENTS ON WOMEN
Romney joined a chorus of critics in blasting Trump after the release of a tape that captured Trump making lewd comments about women.
"Hitting on married women? Condoning assault? Such vile degradations demean our wives and daughters and corrupt America's face to the world," Romney tweeted.
ELECTION NIGHT 2016: ROMNEY CONGRATULATES TRUMP
After Trump won the presidency, Romney tweeted: "Best wishes for our duly elected president: May his victory speech be his guide and preserving the Republic his aim."
NOVEMBER 2016: TRUMP CONSIDERS ROMNEY FOR KEY POST
Just months after blasting Trump, Romney agreed to meet with Trump about becoming Secretary of State. A photo of the two eating dinner together spawned numerous internet memes for Romney's awkward smile and Trump's sheepish grin.
Romney spoke briefly to reporters after the meal, saying he had "increasing hope" that Trump could lead the country to a "better future."
Trump never offered Romney the position, instead choosing former Exxon Mobil CEO Rex Tillerson.
Romney later said in June 2017 that Tillerson was the right choice. Romney said he and Trump were "miles apart" on foreign policy, disagreeing about approaches to Syria, NATO, North Korea, and China.
AUGUST 2017: ROMNEY RENEWS TRUMP CRITICISM
Romney called out Trump for blaming "both sides" following a deadly white supremacist rally in Charlottesville, Virginia.
Trump's words on Charlottesville "caused racists to rejoice, minorities to weep, and the vast heart of America to mourn," Romney wrote on Facebook.
JAN. 5, 2018: TRUMP AND ROMNEY SPEAK ABOUT SENATE
Trump spoke briefly with Romney on Jan. 5 about Utah Sen. Orrin Hatch, who had recently announced he was retiring after four decades, the White House said. Trump had urged Hatch to run again. The conversation came several days after White House press secretary Sarah Huckabee Sanders said she had not discussed Romney's potential candidacy with Trump and could not say whether the president would support him.
JAN. 15, 2018: ROMNEY RESPONDS TO TRUMP'S COMMENT ON IMMIGRANTS
Romney renewed his criticisms of Trump, this time taking to Twitter to respond to comments Trump is reported to have made this week before using a vulgarity to describe African countries and question further U.S. acceptance of Haitian immigrants.
"The poverty of an aspiring immigrant's nation of origin is as irrelevant as their race," Romney tweeted, adding that the message attributed to the president is inconsistent with "America's history and antithetical to American values."