In an op-ed published in the Washington Post Tuesday evening, incoming Republican senator Mitt Romney criticized President Trump’s reliance on divisive rhetoric and suggested that his character flaws have degraded both national unity and America’s standing in the world.

Romney, who will be sworn in Thursday as a U.S. senator from Utah, wasted no time in declaring his opposition to Trump’s strident criticism of American allies and insistence on viewing foreign policy through a strictly transactional lens.

“Trump’s words and actions have caused dismay around the world,” Romney writes, before citing a recent Pew Research poll which found that just 16 percent of people in Germany, Britain, France, Canada and Sweden believe the president of the U.S. would “do the right thing in world affairs” — down from 84 percent just one year earlier.

“The world needs American leadership, and it is in America’s interest to provide it. A world led by authoritarian regimes is a world — and an America — with less prosperity, less freedom, less peace,” he continued.

The former Republican presidential nominee also lamented the effect of Trump’s reliance on charged and often personal attacks against perceived political enemies and the press.

“As a nation, we have been blessed with presidents who have called on the greatness of the American spirit,” Romney writes. “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.”

While he stipulated that he would not offer commentary on all of Trump’s tweets and remarks, as a senator Romney vowed to “speak out against significant statements or actions that are divisive, racist, sexist, anti-immigrant, dishonest or destructive to democratic institutions.”

Update 8:30 a.m.: President Trump responded to Romney’s attack by calling for Republican unity in a Wednesday morning tweet.

Update 10:12 a.m.: Romney’s niece, Republican National Committee Chairwoman Ronna McDaniel, chastised her uncle for criticizing the president in a Wednesday morning tweet sent shortly after the president’s response.