An open letter to Barack Obama: Mr President, it is time. You must speak. Your country needs you.

Eleven summers ago, as you accepted the Democratic nomination for president, you announced: “Tonight, I say to the people of America, to Democrats and Republicans and independents across this great land: Enough. This moment — this election — is our chance to keep, in the 21st century, the American promise alive.”

Of course, we knew then that our nation had deep political differences, and would continue to. Only the most naïve among us would have expected the election of the first African-American president to come about with no backlash.

But would anyone have anticipated that within a dozen years, we would hear another president tell four non-white congresswomen — three of whom were born in this country — to “go back” to where they came from?

Donald Trump’s tweets over the weekend were the basest kind of bigotry, the oldest of racist tropes. And they were a reminder that in the eyes of some, who include our current president, people of colour will never be seen as fully American, regardless of how many generations their families have been part of this nation’s story or how much they achieve as individuals.

President Obama, I understand your reluctance to weigh in on Trump’s latest transgression. You are a respecter of norms, including the one dictating that former presidents steer clear of criticising their successors. And for all the barriers you shattered, racism was never a topic upon which you seemed eager to hold forth.

But silence now is unacceptable. It is time to make yourself heard.

We have long since given up expectations that anyone in Trump’s own party will do or say anything to call him out for what he is. The few who tried at the beginning have capitulated, been driven from office or walked away.

For Trump, racism is not a moral failing; it is a political tactic. None of us can really claim to understand what is in his heart, but the cynicism of his actions is apparent, and it must be exposed for what it is.

No one knows that better than you do. Trump launched his political career by spreading a blatantly racist lie that you had been born in Africa.

At the time, Americans could still be hopeful enough about who we were to write him off as a buffoon. You even joked about it. As Trump sat fuming in the audience at the 2011 White House correspondents’ dinner, you released what you said was your “official birth video”. It was a clip from The Lion King.

But he was just getting started. Birtherism turned out to be Trump’s original sin, the one from which so many others have followed — the 2015 presidential announcement speech in which Trump branded Mexican immigrants as drug dealers and rapists, his call for a ban on Muslims entering the country, his claim that a judge of Mexican descent born in Indiana was incapable of ruling fairly on a case involving Trump University, his declaration that there were “very fine people on both sides” of the deadly clashes between hate-fuelled white supremacists and protesters in Charlottesville.

There was a political calculation behind every one of those vile statements, and as repulsed as we are with his latest racist rant, it is possible to see the calculation that is at work here as well.

Trump knows Democrats will set aside their internal differences, as they should, to rally around the four, who represent their party’s more liberal, more youthful edge. He intends to make representatives Ayanna Pressley, of Massachusetts, Rashida Tlaib, of Michigan, Alexandria Ocasio-Cortez, of New York, and Ilhan Omar, of Minnesota, the face of the Democratic Party, as he wants America to see it.

“These are people that if they don’t like it here, they can leave,” he said yesterday. “They hate our country. They hate it, I think, with a passion.”

The irony was rich, given how incessantly Trump himself criticised this country before he was elected, and how even his campaign slogan — “Make America Great Again” — denigrated both the nation and the progress it has made.

President Obama, what we need more than anything else right now is someone who can lift the country’s sights again. Someone who can charge us, as you once did, with a mission to “march into the future. Let us keep that promise, that American promise, and in the words of Scripture hold firmly, without wavering, to the hope that we confess”.

Find your voice again. Reclaim your legacy. Do it now. It won’t wait until your memoirs.

• Karen Tumulty is a Washington Post columnist covering national politics. She joined the Post in 2010 from Time magazine and has also worked at the Los Angeles Times