Chicago teachers went on strike Monday for the first time in 25 years, leaving 400,000 children off school and creating a political minefield for President Barack Obama in his adopted hometown. The strike in the nation\'s third largest school district is particularly awkward for Obama because the union is fighting his former chief of staff, Rahm Emanuel, who left the White House to run for mayor of the Windy City. Unions are a key source of support for Obama\'s Democrats, and the party has fought to maintain the support of teachers even as Obama pushes for major education reform which would give schools more leeway to fire poor performers. Obama has attacked Republicans for failing to prevent cash-strapped local and state governments from laying off some 300,000 teachers since 2009 and for favoring tax cuts over education funding. Republicans, meanwhile, have blamed public sector unions for massive budget deficits and teachers in particular for the poor performance of many of the nation\'s schools. New Jersey Governor Chris Christie -- a rising Republican star -- has been one of the strongest critics, calling teachers \"lazy\" and \"greedy\" in a showdown over pay cuts and larger class size. Republican White House hopeful Mitt Romney tried to exploit the strike by slamming Obama for cozying up to unions and accusing the Chicago teachers of turning their back on children and \"a city negotiating in good faith.\" \"Teachers unions have too often made plain that their interests conflict with those of our children, and today we are seeing one of the clearest examples yet,\" Romney said in a statement released Monday. \"President Obama has chosen his side in this fight, sending his vice president (Joe Biden) last year to assure the nation\'s largest teachers union that \'you should have no doubt about my affection for you and the president\'s commitment to you,\'\" Romney said. \"I choose to side with the parents and students depending on public schools to give them the skills to succeed, and my plan for education reform will do exactly that.\" The Obama campaign mocked Romney\'s sudden support for Emanuel, with press secretary Ben LaBolt tweeting: \"Chuckles across Chicago as Romney tries to reinvent himself as the city\'s biggest cheerleader after attacking it for the past year.\" \"Romney apparently believes that fewer teachers and larger classrooms is the solution to education challenges,\" LaBolt added in a dig at Romney\'s policies, which would slash school budgets. The White House refused to take the bait and pick a side in the strike. \"We hope that both sides are able to come together to settle this quickly and in the best interests of Chicago\'s students,\" White House spokesman Jay Carney told reporters. A key issue in the Chicago strike is a new form of teacher evaluation that the union says relies too heavily on student test scores and could lead 30 percent of its members -- some 6,000 teachers -- to be dismissed within one or two years. \"This is no way to measure the effectiveness of an educator,\" Chicago Teachers Union president Karen Lewis said in a statement. \"There are too many factors beyond our control which impact how well some students perform on standardized tests such as poverty, exposure to violence, homelessness, hunger and other social issues beyond our control.\" Emanuel castigated the union for choosing to strike rather than to continue to hammer out the few remaining issues at the negotiating table. \"This is a strike of choice and it is the wrong choice for our children. It is unnecessary, avoidable and our kids do not deserve this,\" the mayor said. The school district opened 144 \"children first\" sites at schools where parents could leave their children from 8:30 am until 12:30 pm and -- most importantly -- some of the 338,000 children who rely on the district for free or subsidized meals can get breakfast and lunch. Chicago police also stepped up their patrols to deal with potential problems that could arise with so many kids out of school, as some 25,000 teachers swapped classrooms for the picket lines.