When Donald Trump recently tweeted that four women of color who are members of the U.S. House of Representatives should “go back” to the “corrupt,” “broken,” “crime-infested” countries that they came from, many of us noticed that he had taken a further step down into the racist sub-text of his presidency. Considering Trump’s long history of racist acts and words – from his New York housing projects that excluded blacks in the 1970s, to his racially charged campaign to execute the Central Park Five (who were subsequently exonerated by DNA evidence after serving years in prison), to his “birther” campaign against Barack Obama, to his comments about Mexican rapists, and American Nazis who are “good people,” to his attempted ban of all Muslim immigrants to the United States – he was already deep in the racist slime.

How then could the infamous Tweet of last Saturday really be any worse?

The answer is that it signals the beginning of a presidential re-election campaign based on a pre-Civil War conception of the American people; a campaign based on race and the racialization of political and policy differences. It is designed to fragment the American consensus that for the past 50 years has worked to heal the divisions of slavery and segregation in America and “create a more perfect union,” as the Founding Fathers exhorted us to do when they wrote the Constitution in 1787.

Trump seeks to turn the clock back to 1861, before the Civil War. We must defeat him in this battle and restore the United States to its historic mission of establishing equality under the law.

First, let’s talk about race. Three of the four members of Congress that Trump treated as foreigners and told to go “home” were born in the United States: Ayanna Pressley, an African-American, born on the North Side of Chicago in 1974; Rashida Tlaib, born in Detroit in 1976 to parents who immigrated to America as Palestinian refugees; and Alexandria Ocasio-Cortez, born in the Bronx to a father also born in the Bronx, and a mother born in Puerto Rico. Aside from the fact that Ocasio-Cortez was born in New York, Puerto Rico has been a territory of the United States since 1898, when it was taken from Spain in war, and since 1917, persons born there are recognized as natural born U.S. citizens. As for Ilhan Omar, the only immigrant among the four whom Trump labeled as foreigners, she was naturalized as a U.S. citizen in 2000, at the age of 17.

According to the U.S. Constitution: “All persons born or naturalized in the United States, and subject to the jurisdiction thereof, are citizens of the United States and the State wherein they reside.”

That language is in the 14th Amendment, which was one of three constitutional amendments passed by Congress and ratified by the States during and after the Civil War, for the purpose of extending to African Americans – former slaves – the full rights of U.S. citizenship that had previously been afforded only to whites. But in the wisdom and foresight that has so often (though not always) informed the authors of our Constitution and the amendments to it, the language of the 14th Amendment applied not only to former slaves. It applied, and still does, to all persons, of whatever color, gender, race, religion or national origin, who are “born or naturalized” in the United States. Ayanna Pressley, Rashida Tlaib and Anastasio Ocasio-Cortez were born in the United States, and Ilhan Omar was naturalized in the United States. They are U.S. citizens entitled to “equal protection of the laws,” under that same 14th Amendment which makes them, and us, citizens.

One of the titles we use for the American president is “chief law enforcement officer.” That phrase comes from the clause in the Constitution which states that the President must “take care that the laws be faithfully executed.” He took an oath of office, administered by the Chief Justice of the Supreme Court, to “preserve, protect and defend the Constitution of the United States.” When the chief law enforcement officer of the United States claims that American citizens, members of Congress, are not real Americans and should leave the country, he is violating the equal protection clause – which is a law of the United States – and he is violating his oath of office to preserve, protect and defend the Constitution.

While the subject of impeaching Trump is fraught with political calculations, it is not far-fetched to say that his violation of the 14th Amendment and his oath of office rise to the level of the “high crimes and misdemeanors,” which are impeachable offenses under the Constitution. Andrew Johnson, another racist president, was impeached after the Civil War for his efforts to unlawfully remove officials who supported execution of the laws guaranteeing rights for freed slaves, and for his vituperative attacks on Congress.

In our day it took no time for the public debate on Trump’s words to degenerate into a political brawl, which entirely misses the point of what he did. This political bloodshed is what Trump wanted. In response to the uproar over his “go home” tweet, he attacked Omar and her three colleagues for being anti-Israel and anti-American. The overwhelming majority of Republicans in the House and Senate dutifully followed that line. Now, he is using it for applause at campaign rallies, with his supporters chanting “Send her back” when he mentions Rep. Omar’s name. He tried a new angle on an old issue and he liked what he saw: the same race issue that has been integral to his campaign and his presidency since long before he even ran for office.

And it has also worked to divide his opposition. “Well-meaning moderates” stepped into the trap, cautioning Democrats against going “too far” in reacting to Trump’s racism. Edward Luce, in The Financial Times, wrote, “Mr Trump’s racial tweets have … forced Ms Pelosi to come to the four congresswomen’s defence. He is literally pushing Democrats towards an extremist corner.”

Surely Americans, both elected leaders and average citizens, are intelligent enough to distinguish between endorsement of the leftist positions of Ocasio-Cortez and her colleagues on the one hand, and defense of the underlying constitutional principles of the United States on the other.

And many of them have. One example: despite Trump’s effort to make the issue around Omar and Tlaib about their opposition to Israel and anti-Semitism (claims which have a lot of truth), Jonathan Greenblatt, head of the Anti-Defamation League, the leading Jewish organization for combatting anti-Semitism, said, “Donald Trump using Israel to defend his blatant racism only hurts the Jewish community. He doesn’t speak for any of us. We call on ALL leaders across the political spectrum to condemn these racist, xenophobic tweets and using Jews as a shield.”

Democrats can disagree among themselves about left-wing policies and moderate policies. Republicans and Democrats can disagree between each other about taxes, social welfare and the environment. But the consensus underlying the 14th Amendment, the Civil Rights Act of 1964 and the Voting Rights Act of 1965 about who is an American citizen and what are our rights — to criticize our government and work for change to create a “more perfect union” – is far broader than the individual political opinions we hold about policy issues day-to-day.

When Trump appeals to race, he appeals to no more than his 35% base. The other 65% — Democrats, Republicans and independents alike - have a different vision of America: a vision that has been striving for, and ultimately obtained, majority support, over 200 years of spilled blood and heroic effort: from Benjamin Franklin’s introduction of a petition to abolish slavery in the First Congress in 1790, to the slaughter of the Civil War and its ensuing constitutional amendments, to Lyndon Johnson’s muscling the major civil rights laws through Congress in the 1960s, to the two-term election of Barack Obama in 2008 and 2012.

Opposition to Donald Trump in the presidential election of 2020 is about no less than this: the soul of America and its continued progress. As Abraham Lincoln said at Gettysburg, among the graves of thousands of Union dead, “It is for us the living … to be dedicated here to the unfinished work which they who fought here have thus far so nobly advanced … to be here dedicated to the great task remaining before us — that from these honored dead we take increased devotion to that cause for which they gave the last full measure of devotion — that we here highly resolve that these dead shall not have died in vain.”

When we look to the elections of 2020, this is the vision, and the cause, that we must see. The redemption of the American Dream of a more perfect union, of a better society, of equality under the law. The challenge before us is no less than this, and we must meet it. It is not a time for petty divisions between leftists and moderates; between Democrats and Republicans. It is time for unity to rescue the America that we, and our ancestors and our fellow citizens, have worked so hard to build.

Ted Jonas, an international business lawyer and environmental activist, is a native of Atlanta.