Incoming Sen. Mitt Romney launched a scathing attack on President Trump, saying that the GOP occupant of the White House has not “risen to the mantle of the office.”

Romney, the Utah Republican and former GOP presidential nominee, said Trump’s actions over recent weeks represent a “deep descent “ of his presidency that can’t be saved by Republican-friendly policy actions, such as tax cuts and deregulation.

“On balance, his conduct over the past two years, particularly his actions this month, is evidence that the president has not risen to the mantle of the office,” Romney wrote in an op-ed published in the Washington Post on Tuesday, just days before he will take his seat in Congress.

Romney, who frequently attacked Trump’s character and ethics as a private citizen, criticized the president’s flurry of controversial moves in December, including his decision to pull U.S. troops from Syria, which prompted the resignation of Defense Secretary Jim Mattis.

“The Trump presidency made a deep descent in December,” Romney wrote. “The departures of Defense Secretary Jim Mattis and White House Chief of Staff John F. Kelly, the appointment of senior persons of lesser experience, the abandonment of allies who fight beside us, and the president’s thoughtless claim that America has long been a 'sucker' in world affairs all defined his presidency down.”

The blistering op-ed suggests Romney could replace retiring Republican Sens. Bob Corker of Tennessee and Jeff Flake of Arizona as GOP foils to Trump and defenders of traditional conservatism. While Trump endorsed Romney during the latter's 2012 presidential run, their relationship grew conflicted afterward.

Romney fought against Trump's nomination as the Republican candidate, but was an early candidate for secretary of state in the real estate mogul's administration.

In the op-ed, the incoming senator — like other Republican critics of Trump — said he was comforted early in the president’s term by Trump’s choices of experienced professionals to lead his administration, including Mattis, chief of staff John Kelly, and former national security adviser H.R. McMaster.

He also approved of Trump’s moves to fight China on trade, support criminal justice reform, and appoint conservative judges.

But Romney said Trump’s ethics and leadership have fallen short, prompting the incoming senator to speak out before being sworn in to the Senate on Thursday.

“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. 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 closed the op-ed by previewing how he would approach his role in the Senate, balancing a commitment to the Republican Party with his principles.

“I do not intend to comment on every tweet or fault," Romney said. "But I will speak out against significant statements or actions that are divisive, racist, sexist, anti-immigrant, dishonest or destructive to democratic institutions."