U.S. President Donald Trump and Mitt Romney, the losing 2012 Republican presidential candidate, are engaging in a verbal spat just as Romney is set to claim a Senate seat in the Congress that takes office Thursday.Romney, in a Washington Post opinion article, declared Wednesday that Trump "has not risen to the mantle of the office" during the first two years of his presidency. "To a great degree, a presidency shapes the public character of the nation," Romney wrote. "A president should unite us and inspire us to follow 'our better angels.' A president should demonstrate the essential qualities of honesty and integrity, and elevate the national discourse with comity and mutual respect. As a nation, we have been blessed with presidents who have called on the greatness of the American spirit. With the nation so divided, resentful and angry, presidential leadership in qualities of character is indispensable.""And it is in this province where the incumbent's shortfall has been most glaring," Romney concluded. Trump, never one to ignore an attack on him, retorted hours after the article was published."Here we go with Mitt Romney, but so fast! Question will be, is he a Flake? I hope not," Trump said on Twitter referring to another long-time Republican critic, Arizona Senator Jeff Flake, who is leaving office after declining to seek re-election last year. "Would much prefer that Mitt focus on Border Security and so many other things where he can be helpful," Trump said on Twitter. "I won big, and he didn't. He should be happy for all Republicans. Be a TEAM player & WIN!" Romney and fellow Republican Trump have carried on a love-hate relationship over the last several years.Romney called Trump a "phony" and a "fraud" during the 2016 presidential campaign although Trump later briefly considered naming the one-time Massachusetts governor as secretary of state as he shaped his new government after winning the White House. Trump bypassed Romney, who lost the 2012 presidential election to former President Barack Obama, but last year endorsed his Senate candidacy in the western state of Utah and Romney easily won.In the newspaper column, Romney said that as he becomes a senator, "I will act as I would with any president, in or out of my party: I will support policies that I believe are in the best interest of the country and my state, and oppose those that are not. I do not intend to comment on every tweet or fault. But I will speak out against significant statements or actions that are divisive, racist, sexist, anti-immigrant, dishonest or destructive to democratic institutions."