Teachers in Chicago went on strike for the first time in 25 years on Monday in a bitter dispute with Mayor Rahm Emanuel that is reverberating across the country as the issues at the core of the conflict — teacher evaluations tied to student test scores, a longer school day and other education policy changes — are being hotly debated from Hawaii to Maine.

The fact that the fight revolves around Emanuel, a former chief of staff to President Obama, has pushed the municipal labor fight into prime time and complicated the political calculus. Obama is relying heavily on the support of unions in his reelection bid, and the Chicago strike immediately figured into the landscape of this fall’s political campaigns.

It is also the boldest confrontation yet involving one of a growing number of Democratic mayors who have been pressuring unions to accept policy changes in cities such as Boston, Cleveland and Los Angeles, creating a schism between the Democrats and a traditional ally.

“It’s not just about the negotiations in Chicago,” said Joe Williams, executive director of Democrats for Education Reform. “Because of the visibility of the mayor, this is an important stand for the union. They’re trying to send a message nationally about what teacher unions are going to tolerate from Democratic mayors.”

At a news conference Monday, Emanuel said the strike was “totally avoidable.”

“This is a strike of choice, and it’s the wrong choice for our children and it’s not necessary,” Emanuel said.

The labor dispute lays bare a Democratic Party with significant tensions over the direction of school reform. Major figures such as former D.C. schools chancellor Michelle Rhee and former New York City schools chancellor Joel Klein have pressed for tougher teacher evaluations and an end to “last in, first out” hiring practices that are part of many union contracts. On the other side are labor leaders and other interests convinced that the reforms are union-busting by another name.

While Obama has maintained close ties to teachers, he has promoted policies many of them dislike. They include the Race to the Top grant competition, which rewards states for evaluating teachers in part by how well their students perform on standardized tests.

“There’s frustration and growing resistance to these so-called reforms that are being pushed without any evidence that they work,” said Lisa Guisbond, a policy analyst for the National Center for Fair and Open Testing, which is opposed to high-stakes tests.

Within hours of its start, the strike migrated into the presidential race, providing a tactical opening for Republican candidate Mitt Romney and a sticky political situation for Obama.

Romney underscored the president’s relationship with unionized teachers and, more broadly, organized labor. In a statement, Romney, who has assailed unionized teachers as an obstacle to education reform, also seemed to be taking a page from the playbook of two Republican governors, Scott Walker of Wisconsin and Chris Christie of New Jersey, who made political gains by taking on public employee unions.

“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. “President Obama has chosen his side in this fight.”

Romney cited Vice President Biden’s address last year to the National Education Association’s annual convention, where he assured educators they should have “no doubt about my affection for you and the president’s commitment to you.”

Obama on Monday had nothing to say publicly about the matter embroiling his adopted home town. “The president . . . has not expressed any opinion or made any assessment about this particular incident,” White House press secretary Jay Carney said when asked about the strike.

In Chicago, 26,000 teachers and paraprofessionals picketed in red T-shirts outside empty schools while nearly 400,000 children were left with nothing to do Monday. Scores of churches, community centers and parks welcomed children who needed a safe place to spend the day while the adults continued to try to find agreement at a negotiating table downtown.

The tension in Chicago began before Emanuel was elected. On the campaign trail, he pledged to add 90 minutes to the school day and extend the school year. Chicago is the country’s third-largest public school system but has one of the shortest school days.

Union leaders argued that Emanuel cannot unilaterally extend their workday by 20 percent.

When Emanuel took office last year, the school district faced a $700 million budget shortfall, and he rescinded 4 percent raises for teachers that had been negotiated and settled. He offered bonuses for teachers and schools that waived the contract and adopted a longer school day; the union challenged the move before the Illinois Educational Labor Relations Board.

Emanuel successfully pushed for a new state law that made it harder for the teachers to strike; union members responded by overwhelmingly meeting the new hurdles to authorize a strike. Emanuel and Chicago Teachers Union President Karen Lewis reportedly got into heated exchanges during private meetings. Things went downhill from there.

“Anger. This is about anger,” said Barbara Radner, director of the Center for Urban Education at DePaul University in Chicago. “There is great, great hostility about the mayor right now among the teaching population. They call him ‘Empermanuel.’ He triggered that by saying, ‘I don’t need you. We’re going to have a longer school day.’ ”

Emanuel said Monday that the city’s latest offer to the union was “respectful of our teachers,” noting that it would be a 16 percent pay raise over four years, covering a cost-of-living increase and additional pay for a longer school day. “It does right by our students, and it is fair to our taxpayers.”

One of the remaining sticking points is a new state-mandated teacher evaluation system. The new law calls for student test scores to account for at least 25 percent of a teacher’s job performance rating, and Emanuel wants to increase that to 40 percent over several years.

Lewis says the evaluation system unfairly blames teachers for the poor test scores of students who are struggling with poverty, broken homes, violence and other social ills.

“There are too many factors beyond our control which will impact on how some of our students perform on those standardized tests,” Lewis said Sunday night. “. . . Poverty — which no one wants to talk about — exposure to violence . . . homelessness, hunger and other social issues beyond our control.

“Evaluate us on what we do, not the lives of our children that we do not control.”

Randi Weingarten, president of the American Federation of Teachers, spent the weekend trying to create a bridge between the local affiliate and Emanuel.

When Emanuel was elected, “he wanted to make major changes in Chicago Public Schools and wanted to do it quickly,” said Weingarten, who is flying to Chicago on Tuesday. “Some changes we didn’t agree with, some we agreed with. But changes of that kind of magnitude need to be done collaboratively and correctly, not just quickly.”

Last week at the Democratic National Convention, Emanuel took on another pivotal political role, to bolster fundraising for super PACS backing the party. On Monday, however, he backed away at least temporarily from the fundraising.

Major labor donors to Priorities USA Action, the super PAC supporting Obama, include the Service Employees International Union ($1.5 million) and the National Air Traffic Controllers Association ($1 million), according to Federal Election Commission records. Majority PAC, which is focused on shoring up Democratic control of the Senate, has received nearly $3.3 million from labor groups, including $300,000 from the American Federation of Teachers, records show.

Dan Eggen contributed to this report.