FROM 2:50 a.m. Wednesday until 9:19 p.m. Thursday, Donald Trump was responsible, reassuring — downright presidential. With a single tweet, he then rekindled every legitimate fear of the damage he might do from the White House. And nine hours after that, the president-elect reversed course again — with a contradictory, and statesmanlike, message on Twitter. Whether the opposing messages revealed conflicts within Mr. Trump’s team or within his soul is beyond our ability to judge. But which tendency wins out is a critical question for the country.

The presidential Mr. Trump emerged onstage early Wednesday morning when he promised, in declaring victory, to be a “president for all Americans.” “I say it is time for us to come together as one united people,” he said. After a few hours rest, the Republican victor was on the phone, reassuring key allies. “We will be steadfast and strong with respect to working with you to protect against the instability in North Korea,” he told South Korean President Park Geun-hye, according to her office. On Thursday, Mr. Trump was describing the “great honor” of meeting for 90 minutes with President Obama, calling the president “a very good man” and saying he looked forward to receiving more advice from him.

All of this matters because transitions from one party to the other always are unsettling for the nation and for a world that depends on U.S. leadership — and, given Mr. Trump’s un­or­tho­dox and often ugly route to the White House, particularly so in this case. To say that he needs to offer reassurance is not to suggest that Mr. Trump must abandon policies he campaigned on, though we opposed and will continue to oppose most of those. It does mean that he must recognize that half the electorate voted against him and that many of those voters fear there is no place in Mr. Trump’s vision of America for them or their opposing views.

It was precisely such fears he stoked with his Thursday night tweet responding to protests taking place in numerous cities against his election. “Just had a very open and successful presidential election,” Mr. Trump posted on Twitter. “Now professional protesters, incited by the media, are protesting. Very unfair!” With its unfounded conspiratorial tone, its scapegoating and its sense of grievance, it raised the specter of a divisive, us-vs.-them presidency.

What should the president-elect have said? Precisely what he did say in a tweet at 6:14 a.m. Friday: “Love the fact that the small groups of protesters last night have passion for our great country. We will all come together and be proud!” Mr. Trump is entitled to condemn protesters who turn violent, as a minority of demonstrators did in Portland, Ore. But he should welcome the diversity of views. More than that, he should condemn the ugly episodes of intolerance that have been reported around the country since his election, often carried out in his name.

Some will dismiss Mr. Trump’s second tweet as a transparent effort to mask his true character, only too evident in his first reaction. Others will see the shift over nine hours as an encouraging sign of a man on a learning curve. It lies within Mr. Trump’s power to vindicate the second view.