By shutting down the city’s public schools over a contract dispute, the Chicago Teachers Union (CTU) has left about 350,000 students in the lurch, not to mention their parents, who’ve had to scramble to find safe places to park them during the day. Even if you think the teachers have valid grievances, it’s hard to justify using Chicago’s public school students as pawns in a political test of will with city leaders.

Now in its fourth day, the strike also threatens to throw a monkey wrench into President Obama’s finely tuned campaign machine.

Chicago, after all, is the President’s home town. Its mayor, the sharp-tongued Rahm Emanuel, is Obama’s former Chief of Staff and a key political ally. The CTU, 25,000 members strong, is furious at Emanuel for pushing accountability measures it claims are unfair to teachers. And teachers’ unions are a potent source of votes and money for Democrats.

The stage is thus set for a family feud among Democrats at the worst possible moment – just as Obama seems to be pulling away from Mitt Romney.

So far, Obama has stayed above the fray, though if the standoff isn’t resolved soon, he’ll face mounting pressure to take sides. But the President already finds himself in an awkward position. He and his Education Secretary, Arne DuncanArne DuncanTrump administration is putting profits over students Chicago to make future plans a graduation requirement: report Top Education official resigned over dispute with DeVos: report MORE – a former Chicago schools chief – are ardent champions the very reforms that Emanuel and other big city mayors advocate and teachers’ unions hate: Closing failing schools; expanding charter schools; extending school days to give students extra learning time; adopting common standards; linking teacher pay to performance; and, limiting if not abolishing tenure.

On the other hand, Obama is mainly pursuing a base mobilization strategy, not one aimed at persuading political independents and moderates (that’s Bill ClintonWilliam (Bill) Jefferson ClintonMueller’s probe doesn't end with a bang, but with a whimper Mark Mellman: History’s judgment Congress should massively ramp up funding for the NIH MORE’s job). He can’t afford to give core constituencies reasons to stay home on election day. That’s why speaker after speaker the party’s Charlotte convention extoled the nobility of teachers (who as usual constituted a significant chunk of the delegates) and deplored the GOP tactic of making a political piñata of their unions.

The Chicago strike has illuminated two significant rifts in the progressive coalition. The firstseparates Democratic reformers, including Mayors saddled with dysfunctional school systems and low-income parents who desperately want to get their kids into better schools, from defenders of the education status quo, including teachers unions, school boards and leftish academics who view the reform agenda as a conspiracy by foundations and hedge fund managers to destroy public education.

The second rift runs between private and public sector unionism. The latter is very different from the former, because the employer is, well, the public, not some claque of Romney-esque bonus babies and profit-squeezers. What the public wants is higher quality public services at the least possible cost. These days that means revamping inefficient and antiquated public sector systems, notably broken urban school systems that are failing poor and minority kids. It also entails imposing fiscal discipline on state budgets swollen by generous pay and pension benefits public sector unions have won at the bargaining table.

Chicago is a union town, but it also cherishes its reputation as a city that works. It’s not easy to sympathize with middle class teachers (average salary: $76,000) who demand more pay and job security while resisting more accountability for results. And Rahm Emanual is no Scott Walker, the union-bashing governor in Wisconsin.

As the CTU has learned, however, Emanual is deadly serious about raising the quality of Chicago’s public schools. “This is a strike of choice, and it’s the wrong choice for our children,” he declared at a press conference this week.

Emanuel has pressed for a longer school day (at six hours, Chicago schools have among the shortest) and offered teachers in return a 16 percent pay raise over four years. They say that’s not enough. The main sticking point, however, apparently involves how to measure teachers’ performance and hold them accountable. Like other cities, Chicago wants to grade teachers partly on how their students do on tests.

This is anathema to the CTU, which says it would blame teachers for factors – endemic poverty, broken homes, violence, teen pregnancy – they can’t control. Of course, even great teachers can’t compensate for all of society’s ills. But the assessments also measure students’ year-by-year growth as they move from one teacher to the next. That’s a fairer way to test the impact that individual teachers have on students.

To reform-minded progressives, the CTU’s complaints sound like an alibi – “the kids were already messed up by the time they got here; don’t expect us to fix them.” That alibi is wearing thin, however, as standout urban schools proliferate. These schools, both traditional and charter, are showing show that lifting poor kid’s expectations and academic achievement levels is not mission impossible.

That’s why the best course for everyone – except the Republicans – is for the CTU to pack it in, get back to work and let the city’s kids go back to school.

Marshall is the president of the Progressive Policy Institute.