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When Kamala Harris attacked Joe Biden for opposing federally mandated busing in the 1970s, holding herself up as the beneficiary of a school integration program as a little girl, she created a political rift, but not a substantive one, between two Democratic candidates for president.

Harris and Biden both today support using tools that fall short of federal mandates to try to desegregate schools, despite the stubborn persistence of schools and school systems that in many cases can still be called separate and unequal.

Instead, they want to give local districts grants to reevaluate their residential zones and develop programs to attract students from outside those zones. That's the politically cautious and pragmatic course, and perhaps the wise one, but it's hardly radical.

I for one hope either a reporter or the Trump campaign revisits the busing rift and attempts to drive a wedge in the perceived Biden-Harris divide, because the sitting president has tried his darndest to set back even cautious attempts to mix Americans by racial and ethnic background.

Trump is apoplectic about the fact that Biden wants to bring back an Obama-era rule called Affirmatively Furthering Fair Housing, which his administration has summarily rescinded. The rule tells jurisdictions that receive federal housing funds that they have to assess what patterns of housing discrimination they have — and then develop plans to diminish them.

Modest. Moderate. But at least doing something more than pay lip service to the fact that America remains painfully divided along racial lines, in no small part because government, over generations, wanted to keep it that way.

A hyperventilating Trump says the rule will "destroy" or "abolish" the suburbs. Desperately speaking to "the Suburban Housewives of America," most of whom have been properly appalled by his divisiveness, he has said, "Biden will destroy your neighborhood and your American Dream. I will preserve it, and make it even better!"

He clunkily, almost comically tweeted, "I am happy to inform all of the people living their Suburban Lifestyle Dream that you will no longer be bothered or financially hurt by having low income housing built in your neighborhood … Your housing prices will go up based on the market, and crime will go down."

As if all that wasn't enough, he added, "People have worked all their lives to get into a community, and now they're going to watch it go to hell. Not going to happen, not while I'm here."

The president of a nation that's more diverse than ever but still terribly divided is not-at-all-subtly telling white families that the poorer Black and brown families who might wish to live in their proximity in newly built affordable housing are going to ruin their lives.

His watered-down version of the Obama-era rule would effectively diminish the 1968 Fair Housing Act to only prohibiting racial discrimination when those who have written the rules expressly admit that racial bias is their intent. Goodbye teeth, hello gums.

To be clear: This one federal rule will not swiftly or boldly sweep away stubborn, cemented legacies of unfair treatment, equalizing wealth and educational opportunity. It won't on its own reverse residential segregation patterns that in many cases are reinforced by far-harder-to-get-at human behavior. But it at least opens the door to schools that are more racially mixed, to health care systems that serve people more equally, to bridging huge economic gaps, to a fairer justice system.

Trump and Mike Pence would slam that door shut, pointing at lower-income families, many of whom happen not to be white, as personal threats to the suburban American dream. This would raise, not lower the many invisible walls that still divide America.

Biden and Harris should relish the chance to revisit their school busing dust-up. Then, they should turn the conversation to Trump, forcefully calling him out for turning back a meaningful federal attempt to prod local communities to enable the construction of housing where more people can live, lessening America's huge housing affordability problems while giving children and families more opportunity to mix.

Trump, who can only rule a nation where we see each other as threats, needs America to remain split between the cities he insists are Godforsaken hellholes and their supposedly safe and relatively homogenous suburbs. Biden and Harris can calmly, finally lead us to a better future.

Josh Greenman is the New York Daily News editorial page editor.