President Obama is staying neutral as a massive strike in Chicago pits teachers unions against Mayor Rahm Emanuel, Obama's former chief of staff and a prominent fundraiser in the president's 2012 re-election push.

White House press secretary Jay Carney said only that Obama's "principal concern is for the students" and that "we hope that both sides are able to come together to settle this quickly and in the best interests of Chicago's students."

"The president said, as he should, that this is a local dispute," American Federation of Teachers president Randi Weingarten told CNN on Tuesday. "This is going to be solved at the bargaining table between the mayor and the teachers union," Weingarten added.

That didn't stop Republican nominee Mitt Romney from assailing the president for siding with unions, a frequent target of Romney's as he calls for education reforms.

"President Obama has chosen his side in this fight, sending his vice president 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 in a statement, referring to a speech that Joe Biden delivered to the National Education Association in 2011.

The reality is more complicated. It is true that teachers unions have been and remain a critical bastion of support for Democrats, and the National Education Association voted to endorse Obama's re-election push more than a year ago. 

But that vote came with widespread doubts about Obama's education agenda, which has included measures -- notably his support for merit pay and for more rigorous teacher evaluations that are tied to student test scores -- that are at the heart of the unfolding dispute in Chicago. The president has signaled his support for sweeping reform, speaking of the need for "radical change."

Education Secretary Arne Duncan has been a driving force in pushing this agenda, manifested in a competitive grant program called Race to the Top and in the administration's offering waivers to exempt states from key provisions in the No Child Left Behind Law. In both cases, states in pursuit of federal dollars or a reprieve from No Child Left Behind had to embrace reforms prized by the administration.

Prior to becoming Obama's education chief, Duncan served as chief executive of Chicago Public Schools -- and in that capacity, he developed a reputation for pushing the kinds of reforms Chicago teachers are disputing. That links him to Emanuel, who has argued that teachers are embarking on an unwarranted "strike by choice." 

"It's the wrong choice for our children and it's not necessary. Totally avoidable," Emanuel said on Monday.

Emanuel has backed teacher performance ratings tied to measures of student progress like test scores, something that is a key sticking point in the current contract disputes. The push to better grade teachers has informed education reform battles across the country, and while reforms say it is needed to hold teachers accountable, unions have argued that such evaluations are overly rigid and do not account for factors outside of the classroom, like the difficulty of educating impoverished students who are less equipped for school.

"This is no way to measure the effectiveness of an educator," Chicago Teachers Union president Karen Lewis said in a statement. "Further 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. "

Another factor is job security. The teachers' union wants measures that allow teachers who have been laid off or lost their jobs through school closures to get priorities for new job openings. On the other side of the argument are education reformers who say teacher seniority rules protect educators who perform poorly. Obama and Duncan have backed that notion.

"Instead of a lifetime guarantee, tenure needs to be a recognized honor that signifies professional accomplishment and success," Duncan said in announcing a proposed $5 billion teacher quality fund in February. "And we need a system of due process to deal fairly with those who are not up to the challenge."