Democrats have a great opportunity in 2020. Their nominee will face off against an unpopular president who struggles to get past the investigation into Russian interference in the previous election. The crisis on the southern border might cut both ways politically, but for the moment provides candidates with lots of opportunities to paint President Trump as heartless. A new Democratic majority has made their agenda at least somewhat more relevant than in the previous session of Congress, an advantage that House Speaker Nancy Pelosi (D-Calif.) can use to force votes that will paint Trump's fellow Republicans into political corners.

All Democrats have to do is stay focused on the present and future, and refrain from scaring voters more than Trump does. Instead, their first round of debates produced a heated argument over a busing — a controversial and deeply unpopular social policy from decades ago aimed at solving an entirely different set of problems than Americans face at the moment. Democrats are giving the distinct impression that they might bring back busing, and this threatens to undo their successes in 2018's midterms and their 2020 momentum against Trump.

Sen. Kamala Harris (D-Calif.) raised the issue in the debate as a planned attack on frontrunner former Vice President Joe Biden, who had made himself vulnerable to such an attack by claiming success in dealing with segregationists in the Senate. Harris declared it "hurtful" that Biden had appeared to legitimize those segregationists. "And it was not only that," Harris continued, "but you also worked with them to oppose busing. And you know, there was a little girl in California who was part of the second class to integrate her public schools and she was bussed to school every day and that little girl was me."

The remark infuriated Biden, who shot back that he "was a public defender" in part to fight for civil rights, adding in a criticism of Harris' resumé, "I didn't become a prosecutor." Biden then clarified that he didn't oppose busing when local school districts used it as a tool for desegregation, but that he opposed federally mandated busing. "You would have been able to go to school the same exact way," Biden noted, "because it was a local decision made by your city council."

If Biden was infuriated, everyone else must have wondered what the point of Harris' sudden endorsement of busing meant. The Washington Post's Richard Cohen argues that it means Democrats want to really test the limits of never-Trump pledges. Cohen reminded readers that forced busing touched off populist revolts in major cities of the kind normally associated with, well, Trump himself. "Mobs formed in the streets, and the cops had to be called," Cohen recalls, although school-district boundaries left some urban districts with no other tools than busing to meet federal desegregation mandates. Regardless of whether it produced some individual success stories, such as Harris', busing remained tremendously unpopular until its bitter end.

What might be more important is where busing would be unpopular today. One major if unintended consequence of forced busing was an acceleration of "white flight," as parents opted out by moving their families from the cities to the suburbs. Not only did the outflow of more affluent families, including black and Hispanic families, starve school districts of children to bus into those schools, it also drove capital and a large part of the urban tax base out with them, along with the resources to address failing schools.

As those school districts exist today, busing wouldn't solve the problem — much as it didn't in the 1970s. To get the influx of enough non-minority students into those schools to make a meaningful impact on the de facto segregation left behind as a consequence of white flight, a Democratic administration would have to get the students from the suburbs. It would also take the Supreme Court overturning its precedent in Milliken v. Bradley, which limited busing to within school districts, but numerically the suburbs would be the only source for busing exchanges on the scale needed.

Therein lies the 2020 risk. Cohen mentions a number of losing issues bandied by Democrats in the past few weeks, but none of them are as potentially explosive as busing. Slavery reparations are hypothetical at best, and none of its small number of advocates have any realistic suggestions as to how it would work. Medicare-for-all might be more developed as a policy, but at least thus far it's being sold as only an indirect change to the way health care gets reimbursed. Busing, however, sends a clear and unequivocal message to suburban voters: Democrats plan to send their children far from home for their education, again.

FiveThirtyEight's Geoffrey Skelley was one of the first to quantify where Democrats made their impressive electoral gains — the suburbs. "Seats that leaned Republican but weren't in rural areas proved to be pretty big pickup opportunities for Democrats," Skelley wrote a few days after the election, "and may be part of a larger story on the growing divide between urban and rural America."

Democrats picked up three dozen House seats in the midterms, three-quarters of which came from suburban districts — nearly evenly split between "dense suburban" and "sparse suburban" districts. A dozen of those districts voted for Trump and Mitt Romney in the previous two presidential elections, and another four for Trump and Barack Obama. It was a potential measure of Trump fatigue that appeared to be a grave omen for the GOP in 2020.

With busing getting resurrected as a Democratic talking point, this suburban momentum could stall entirely. In fact, it could backfire even further than just the 2018 gains that made Pelosi speaker again. Democrats went into the midterms with a 56/27 advantage in "dense suburban" House districts and came out the other side with a 68/15 lead. The fears of a new era of busing would likely hit those districts harder and could give Trump and the GOP more momentum — and permanently make Democrats a party whose appeal is limited strictly to urban cores. That would not only lose the presidency, it would likely hand back the House and put the Senate out of reach.

"The candidates are campaigning in an America of their own imagination," Cohen lamented, "pander[ing] to the extremes of the early caucus and primary states, thinking that they can seduce the middle later on down the road." Don't think for a moment that voters haven't seen this as well, especially the voters that Democrats can't afford to lose — and who already voted for Trump in 2016. Democrats are succeeding only in becoming scarier than the status quo.