A little over a year into Donald J. Trump’s presidency, on the eve of his first State of the Union address, here is what we know. To the extent that his presidency is failing—and Mr. Trump’s historically low approval ratings in the 30s suggest many people believe it is failing—it is not his policies that are to blame but rather his character.

Mr. Trump is impulsive, superficial, crass, cruel, unpredictable and undisciplined. In the 2016 election, many voters overlooked these flaws in the belief that he would study up, grow up and become more presidential once in office. But that has not happened, and his behavior is unnerving even some of his supporters.

I have spent the last year speaking with voters in counties that were pivotal to Mr. Trump’s election victory. The most common refrain I have heard from Trump supporters and critics alike is that they wish he would cut out his personal attacks on political opponents.

Trump supporters and critics alike wish he would cut out his personal attacks on political opponents.

“Mr. President, please stop the negative tweets,” Mark Locklear of Robeson County, N.C., wrote me last July when I asked him what advice he would give at the six-month mark of the new presidency. Mr. Locklear voted for Mr. Trump after twice voting for Barack Obama. He gave the president an “A” on policy but lowered his overall grade to a “C” because of Mr. Trump’s injudicious use of Twitter.

I received similar responses when I surveyed people on the anniversary of Mr. Trump’s inauguration. Pramit Patel, an Indian-American hotel owner also in Robeson County, said he voted for Mr. Trump out of an appreciation for his business acumen. While pleased with the new tax reform law, Mr. Patel labeled the president’s first year “erratic” and is afraid his bellicosity will provoke an international disaster.

Lois Morales of Orange County, Calif., said she thinks Mr. Trump is performing well “except for not shutting up. His actions need to speak louder than his words.”

“His actions need to speak louder than his words.”

Mr. Trump’s character flaws obscure the popularity of many of his administration’s policies. A recent Harvard-Harris poll found that most voters support a more restrictive, merit-based immigration system, and numerous polls show that a majority of Americans approve of Mr. Trump’s handling of the economy. And Mr. Trump’s selection of Neil Gorsuch to the Supreme Court was praised even by some political opponents.

But Mr. Trump’s poor character is not just costing him style points. It is hindering his ability to govern.

To many on Capitol Hill, the president is simply too unpredictable to trust. Ahead of the government shutdown in January, Mr. Trump made wildly contradictory statements about where he stood on the funding impasse, leaving many lawmakers bewildered and annoyed. “The president’s role [in negotiations to keep the government open] has been extraordinarily confusing,” said Senator Susan Collins, a Republican from Maine.

Mr. Trump was even more unpredictable in the related debate over the fate of DACA recipients, immigrants brought illegally to the United States as children. One day Mr. Trump would signal support for a compromise that granted them legal status. The next day he would take a much harder line, use racially charged language or mock a Democratic senator involved in the meetings, calling Minority Leader Chuck Schumer “Cryin’ Chuck.” Mr. Trump’s capriciousness led Mr. Schumer to say that dealing with him is “like negotiating with Jell-O.”

While the government shutdown should not be pinned on Mr. Trump, the self-styled master negotiator did not help matters. The government reopened after two days not because he negotiated some master deal but because he largely stayed out of the debate and off Twitter, while Congress worked out a temporary deal to turn the lights back on.

Mr. Trump’s erratic behavior and habit of publicly attacking members of his own cabinet are also draining morale in the executive branch and have led to a record number of White House departures in his first year. As one State Department official told a reporter: “One can never be sure whether the policies we’re working on will be supported by the president or not. It creates a great deal of uncertainty and obviously further harms morale in an environment in which morale is already very low.”

The confusion, distrust and ill will caused by Mr. Trump’s poor character are undermining his presidency. Whether they also make him a one-term president remains to be seen.

This article appears in February 19 2018.

Daniel Allott is the author of The Race to 2020: The People and Places That Will Define a Presidency and formerly the Washington Examiner’s deputy commentary editor.