With his “tender age” shelters to house the babies ripped from their migrant parents at the U.S. border, President Trump is doing it again. He’s pushing the envelope of human decency, waiting to see how much he can get away with before persisting forward. His signing an executive order temporarily ending the reprehensible separations is a prime example. That the “zero tolerance” policy remains intact means the United States will warehouse thousands of illegal migrant families together in Trump prisons on the southern border.

Once again, our sense of decency has been eroded. What appears to be relief or responding to backlash is merely a shade better than the horrific policy that sparked the outrage. America is the frog in the proverbial pot, and the water is boiling.

The first time Trump pushed the envelope was in 2011, when the Queens-born Manhattan builder championed the racist lie that then-President Barack Obama had not been born in the United States. Trump learned an indelible lesson then. He could say and do the most outrageous, norm-busting things without fear of consequence. Joshua Green pointed this out in his book “Devil’s Bargain: Steve Bannon, Donald Trump, and the Nationalist Uprising.”  After highlighting the milquetoast response of the Republican Party establishment to this overt racism and denigration of a sitting president, Green wrote, “The lesson Trump took away was that the party gatekeepers, who were privately appalled at his behavior and did not want him in the race, would pose no threat to him at all if he decided to run.”

Trump has been pushing the envelope ever since.

“They’re rapists,” Trump said when he announced his candidacy on June 16, 2015, about Mexican immigrants crossing the border illegally. A month later, he denigrated the service of Sen. John McCain (R-Ariz.) by saying that the former prisoner of war who was shot down over Hanoi during the Vietnam War was not a hero “because he was captured.” By December 2015, Trump was calling for a “total and complete” ban of Muslims entering the United States. He questioned the independence and impartiality of federal Judge Gonzalo Curiel by saying that he “happens to be, we believe, Mexican” in May 2016. Curiel, who was then hearing a case against Trump University, was born and raised in Indiana. Trump spent days hurling insults at Khizr Khan and his wife, the Muslim American Gold Star parents whose son, Humayun Khan, gave his life serving with the Army in Iraq. Khan dressed down the future president of the United States during a speech at the 2016 Democratic National Convention. Audio from a 2005 “Access Hollywood” interview unearthed by The Post’s David Fahrenthold in October 2016 let the American people hear Trump brag that fame allowed him to sexually assault women. “You can grab them by the pussy,” he said to laughter.

Despite all of that, and so much more I haven’t mentioned, Trump was elected president. His amoral awfulness has continued unbounded in the Oval Office. This time, with greater consequences for our democracy and the rule of law.

Trump fired James B. Comey as director of the FBI in May 2017 over the agency’s investigation into Russian interference in the 2016 presidential election. Trump continued his assault on the FBI by having Attorney General Jeff Sessions fire Andrew McCabe as deputy director of the FBI, two days before his official retirement. The president threatens special counsel Robert S. Mueller III on a near-daily basis in an effort to delegitimize his Russia probe. Nevermind that many Trump campaign officials have been indicted by Mueller, including Paul Manafort, the former campaign chairman who has been in jail since June 15 for crimes committed AFTER he was indicted by Mueller.

And Trump persists in seeding the ground for his all-but-certain pardon of former associates ensnared in the Mueller probe. The August pardon of former Maricopa County, Ariz., sheriff Joe Arpaio was a canary in the coal mine. Arpaio is an unrepentant xenophobe and racist who was convicted of criminal contempt a month earlier for defying a 2011 court order to stop detaining folks because he suspected they were undocumented. Trump’s pardon of Dinesh D’Souza, the racist conservative commentator who pleaded guilty to campaign finance violations, and his floating that he might do the same for Martha Stewart and Rod Blagojevich, was more message-sending last month.

Apologies for the exhaustive recounting of history, but it is necessary to see how much hotter the water we’re in has become since Trump has been in our political life.

Adding to our scalding environment is a Republican-controlled Congress abrogating its constitutional power and duty to serve as a check on the executive by cowering in fear of a man unfit and unqualified for the office he holds. A man who ceded the moral authority of his office by giving rhetorical aid and comfort to white supremacists marching on Charlottesville in August. A man who whipped up the racist fervor of his base on Tuesday by ranting on Twitter about immigrants who “infest our country” and reprising later in a speech one of his greatest xenophobic hits by saying that nations were “not sending their finest,” “not their best” to the U.S. border. No doubt he was egged on by Sessions and Stephen Miller, the president’s policy adviser and resident white supremacist on staff, now that Steve Bannon is gone. The lusty applause and cheers for Trump’s red-meat oration confirmed for me that there is a sizable chunk of this nation that is fine with what has been happening.

They seemingly are fine with tearing families apart. Fine with pictures of parentless children standing terrified in the shadow of law enforcement officers. Unmoved by the wails of little children, one begging for “Papa, Papa,” while another pleads to call an aunt. Uncaring about the child abuse being waged with taxpayer dollars. Cold in the face of news that the government has no plan for reuniting these children with their families. Stone-hearted at the news that the United States of America is opening prisons for babies and toddlers separated from their parents. And unblinking in their attitude probably best summed up by the moral void that is Corey Lewandowski.

“Womp, womp,” the hotheaded former Trump campaign manager said in response to former Hillary Clinton campaign operative Zac Petkanas, who was relating a story about a 10-year-old with Down syndrome being separated from her mother. Disgusting. Pathetic. Appalling. Shameful. Typical. Trumpian.

The nation and the world have been aghast at the immoral treatment of immigrant families coming across the southern border. Without question, they are breaking the law. I’m not arguing that they should not be held accountable in an appropriate manner. But what Trump and his band of heartless minions are doing tears at the mythical yet inspirational fabric of the United States. For generations, people have risked their lives to come to this nation. Our founding ideals were and remain their enduring beacon of hope and freedom. We cannot let that beacon be snuffed out — for them or for us.