If you haven't yet seen it, you really need to watch Thursday night's Democratic convention speech by Khizr Khan, the father of Army Cpt. Humayun Khan, a Muslim immigrant who was killed in action in Iraq in 2004 while protecting his unit from a car bomb.

Khan demanded to know whether Donald Trump had even read the Constitution, pulled out his pocket copy, and offered to lend it to Trump.

I watched this moment live and was awed by it. I watched it again Friday morning, and I cried.

We are having an election that is about whether we, as a nation, value people like Khizr and Humayun Khan. Whether they are real Americans. Whether we will define our nation by shared values, as both parties have claimed we do for decades, or by ethnicity, as Donald Trump would have us do.

Of course, Trump supporters object to the claim that this is what Trump wants. Donald Trump is talking only about immigrants living in the US illegally, they say. He's talking only about barring foreign Muslims. David Duke, the former Ku Klux Klan leader, may love Trump, but Trump's fans will insist that the Republican nominee's politics are distinct from white supremacy.

This is a load of nonsense, as we can all tell by Trump's attacks on "Mexican" Judge Gonzalo Curiel and by his demands for President Obama's birth certificate. Trump's concept of the nation he speaks for is not about values or citizenship or even birthplace. It is about ethnicity.

If you are a white model from Europe, like Antonio Sabato Jr. or Melania Knauss, you are welcome in Trump's America. If you are a brown or black person, you are suspect, even if you are a citizen, and even if you were born in Indiana or Hawaii (as in the cases of Curiel and Obama).

This is the philosophy of a major-party candidate for president, who has most of his own political party lined up behind him. It is enraging, it is scary, and it is sad. And I cried Friday morning because it was even necessary for someone to stand up at a party convention and explain why that candidate is wrong.

I am angry at Donald Trump, and I am angry at the people who voted for him. But most of all I am angry at the senior Republicans who are standing by and acting as if this is fine — endorsing him in the belief that he will lose but that standing together will stem the loss of congressional seats, or endorsing him in the hope that he will grow up if he wins.

I genuinely thought mainstream Republican leaders knew better, that they understood there are matters more important than fiscal policy, and that if a candidate were terrible enough, they would reach a point at which they realized their responsibilities to their country exceeded those to their political party.

I did not expect people like House Speaker Paul Ryan to behave so indecently as to line up behind this hateful man, who does not even agree with them on public policy. I was naive, and I am sad, because it means we have a less durable democracy than I thought.