Sen. Susan Collins, R-Maine speaks during a news conference on Capitol Hill in Washington, Tuesday, June 21, 2016. (Evan Vucci/AP)

The writer, a Republican, represents Maine in the Senate.

I will not be voting for Donald Trump for president. This is not a decision I make lightly, for I am a lifelong Republican. But Donald Trump does not reflect historical Republican values nor the inclusive approach to governing that is critical to healing the divisions in our country.

When the primary season started, it soon became apparent that, much like Sen. Bernie Sanders (I-Vt.), Mr. Trump was connecting with many Americans who felt that their voices were not being heard in Washington and who were tired of political correctness. But rejecting the conventions of political correctness is different from showing complete disregard for common decency. Mr. Trump did not stop with shedding the stilted campaign dialogue that often frustrates voters. Instead, he opted for a constant stream of denigrating comments, including demeaning Sen. John McCain’s (R-Ariz.) heroic military service and repeatedly insulting Fox News host Megyn Kelly.

With the passage of time, I have become increasingly dismayed by his constant stream of cruel comments and his inability to admit error or apologize. But it was his attacks directed at people who could not respond on an equal footing — either because they do not share his power or stature or because professional responsibility precluded them from engaging at such a level — that revealed Mr. Trump as unworthy of being our president.

[When it comes to Trump, a Republican Treasury secretary says: Choose country over party]

My conclusion about Mr. Trump’s unsuitability for office is based on his disregard for the precept of treating others with respect, an idea that should transcend politics. Instead, he opts to mock the vulnerable and inflame prejudices by attacking ethnic and religious minorities. Three incidents in particular have led me to the inescapable conclusion that Mr. Trump lacks the temperament, self-discipline and judgment required to be president.

The first was his mocking of a reporter with disabilities, a shocking display that did not receive the scrutiny it deserved. I kept expecting Mr. Trump to apologize, at least privately, but he did not, instead denying that he had done what seemed undeniable to anyone who watched the video. At the time, I hoped that this was a terrible lapse, not a pattern of abuse.

The second was Mr. Trump’s repeated insistence that Gonzalo Curiel, a federal judge born and raised in Indiana, could not rule fairly in a case involving Trump University because of his Mexican heritage. For Mr. Trump to insist that Judge Curiel would be biased because of his ethnicity demonstrated a profound lack of respect not only for the judge but also for our constitutional separation of powers, the very foundation of our form of government. Again, I waited in vain for Mr. Trump to retract his words.

Third was Donald Trump’s criticism of the grieving parents of Army Capt. Humayun Khan, who was killed in Iraq. It is inconceivable that anyone, much less a presidential candidate, would attack two Gold Star parents. Rather than honoring their sacrifice and recognizing their pain, Mr. Trump disparaged the religion of the family of an American hero. And once again, he proved incapable of apologizing, of saying he was wrong.

I am also deeply concerned that Mr. Trump’s lack of self-restraint and his barrage of ill-informed comments would make an already perilous world even more so. It is reckless for a presidential candidate to publicly raise doubts about honoring treaty commitments with our allies. Mr. Trump’s tendency to lash out when challenged further escalates the possibility of disputes spinning dangerously out of control.

I had hoped that we would see a “new” Donald Trump as a general-election candidate — one who would focus on jobs and the economy, tone down his rhetoric, develop more thoughtful policies and, yes, apologize for ill-tempered rants. But the unpleasant reality that I have had to accept is that there will be no “new” Donald Trump, just the same candidate who will slash and burn and trample anything and anyone he perceives as being in his way or an easy scapegoat. Regrettably, his essential character appears to be fixed, and he seems incapable of change or growth.

At the same time, I realize that Mr. Trump’s success reflects profound discontent in this country, particularly among those who feel left behind by an unbalanced economy and who wonder whether their children will have a better life than their parents. As we have seen with the dissatisfaction with both major- party nominees — neither of whom I support — these passions are real and the public will demand action.

Some will say that as a Republican I have an obligation to support my party’s nominee. I have thought long and hard about that, for being a Republican is part of what defines me as a person. I revere the history of my party, most particularly the value it has always placed on the worth and dignity of the individual, and I will continue to work across the country for Republican candidates. It is because of Mr. Trump’s inability and unwillingness to honor that legacy that I am unable to support his candidacy.