Thousands, maybe tens of thousands, of people rallied to support the Chicago Teachers Union, as its representatives moved closer to a negotiated agreement with the city that would bring them back to work–and send 350,000 students back to school.

Mayor Rahm Emanuel promised education reform when he took office, and his ideas included a longer school day, a longer school year (currently, Chicago schools are open for 170 days a year, short by US standards, and the United States has the shortest school year of any rich country), more charter schools, and more vigorous, test-based (value added) evaluation of teachers.

The teachers union has obvious gripes: they want to be compensated for more work time, are reluctant to cede jobs to the charters–which haven’t generally demonstrated greater effectiveness, and they want input on their evaluation, knowing that the value-added measures have been wildly inconsistent.

Taking the teachers on strike was a high risk move.  Parents want the best for their kids, and generally want their teachers treated fairly, but they also want their kids in school.  Union leaders know their support will erode over time.  Mobilizing a large turnout at a rally is an indication that they have support–at least now.  And even the lowest estimates of the turnout far exceed the demonstrations at the Republican and Democratic conventions.  The New York Times reports supporters came from Wisconsin and Minnesota, as well as the neighborhoods of Chicago.

But even if the teachers turned out the 50,000 people they hoped for–or more–that doesn’t necessarily reflect their support among the voters of Chicago.  The union mobilized and demonstrated the intense commitment of its supporters, which can be powerful.  But Mayor Emanuel is at least as concerned with a much larger number of people in the City who won’t show up at a rally, but might turn out to vote.

In the streets and parks, the intensity of commitment matters a great deal, but in other kinds of politics, numbers trumps strong commitment.  Organizers have to take risks about translating their support to different kinds of political contests.  Bet that every Chicago Teachers Union organizer remembers how vital and exciting the demonstrations in Madison, Wisconsin were, just over a year ago.  People who cared a great deal marched through the streets in the winter, and slept in the Capitol.  Feeling the tremendous enthusiasm of their side, they staged a recall of Governor Scott Walker–and suffered a harsh and demoralizing defeat, as larger numbers of people who were less engaged had a chance to weigh in.

The CTU will try to get as much as it can in negotiations now, and the demonstration adds some leverage.  The City is eager to get its children back in school, and union leaders know their greatest advantage is now–or soon.  They’ll be eager to claim a victory before the less committed get involved.