When her union endorsed Barack Obama for president in 2008, retired Montgomery County teacher Jane Stern wrote checks to his campaign and spent hours calling voters in swing states to support a Democrat she though would stand strong for public schools and break from a federal education policy of “testing, testing, testing.”

Three years later, all the standardized tests are still there. In some places, they are beginning to be used to fire teachers. Lately, Stern said, the solutions to all of public education’s troubles seem to boil down to a refrain: “Blame it on the teacher who works her tail off for 14 hours a day.”

On Monday, the nation’s largest teachers union will vote at its annual convention here in Obama’s home town on a proposal to endorse his reelection. Stern came to Chicago to vote no.

Her frustrations echo those of many in the National Education Association, who chafe at the president’s support of public charter schools, which compete with traditional public schools, and policies that link teacher tenure and pay in part to growth in student test scores.

These rank-and-file skeptics of Obama, who ordinarily would be eager foot soldiers in a Democratic presidential campaign, acknowledge that the president helped steer tens of billions of dollars to schools after the 2008 financial crisis to prevent teacher layoffs. But they say they are turned off by an administration agenda on school reform that often wins praise from influential Republicans.

Vice President Biden acknowledged their grievances in a speech to the convention Sunday and pledged that Obama would work for the middle class and organized labor.

“We will fight alongside you, we will fight for you and occasionally, in the privacy of the family, we’ll fight with you,” Biden said. “But this is about the same fundamental vision for this country.”

Union leaders said the endorsement is likely to pass, and even some teachers who are highly critical of the president said they would, with reluctance, vote for Obama next year. But in politics, enthusiasm matters. It is an open question how much energy teachers will devote to Obama’s reelection if they have mixed feelings about his record. For that reason, the NEA convention may be telling.

Despite the army of blue T-shirts proclaiming “United We Stand, Divided We Fall” on display here at the McCormick Convention Center, the endorsement question has fractured the 3.2 million-member union.

Debate began as soon as the union’s leadership recommended the endorsement in May. Protests erupted on teacher blogs and in faculty rooms. Some local affiliates, including the one in Baltimore County, took a stand against it. An 800-member policy council of the California Teachers Association narrowly opposed the endorsement in a nonbinding vote in advance of the convention.

Dean Vogel, president of the California affiliate, said the vote reflected disappointment and anger with the Education Department more than how teachers feel about the president.

As they gathered in Chicago, some delegates vented about Obama’s support for charter schools one moment and reminisced about his inauguration night the next. Teachers signed up for trolley tours to see Obama’s house and bid on an autographed poster of the president in a silent auction.

For some, the weightier issue was not whether to support Obama, but when.

“We don’t want our endorsement to be taken for granted,” said Dionna Ricks, a Montgomery elementary teacher.

A spokesman for the American Federation of Teachers, the second-largest teachers union, said the AFT will solicit more input from its 1.5 million members before making an endorsement.

In another warning sign for Obama, a network of teachers and parents is planning a “Save Our Schools” rally outside the White House on July 30 to protest policies that organizers say have “demoralized teachers” and reduced education to “nothing more than test preparation.”

“Teachers are being targeted,” said Ken Bernstein, a Prince George’s County teacher who is helping to coordinate the march. “And they are finally coming out of their classrooms and getting interested” in organizing a political response.

Education Secretary Arne Duncan, a former Chicago schools chief who is close to Obama, often talks about the importance of collaborating with unions. The Education Department organized a labor-management conference in Denver in February to promote that message. But NEA delegates approved a measure Saturday that declares the union is “appalled” at Duncan’s actions on various matters. For instance, the delegates contended that he has focused “too heavily” on competitive grants rather than on providing funding to help all students.

“We acknowledge differing views and interpretations among some education labor leaders around the administration’s education agenda,” Peter Cunningham, a department spokesman, said in a statement. “But on the whole our partnership with labor is having a positive impact on student learning and the teaching profession. We look forward to continuing to work with the NEA in the months and years ahead to further advance education reform.”

Duncan meets monthly with NEA President Dennis Van Roekel, a line of communication the union did not have with the George W. Bush administration. Still, many union members wonder how much influence they have. They say they were humiliated last year when Obama expressed support for a Rhode Island school board that had voted to fire the teachers of a struggling high school. (The faculty was later rehired.)

To win over skeptics, Van Roekel made a pitch for solidarity in national conference calls and state caucuses in the weeks and days leading up the convention. He reminded teachers about Obama’s commitment to pre-kindergarten programs, Pell grants for college students from low-income families, and massive federal aid that in 2009 and 2010 saved teacher jobs from coast to coast.

“I’m willing to fight . . . with the administration on policies with which we disagree,” Van Roekel told Maryland delegates Thursday. “What I fear is going back to the place where we are fighting on the very destination of America.”

In many states, teachers unions have been on the defensive as Republican governors and lawmakers have pushed to end tenure and curb collective bargaining rights. In February, Obama spoke out against what he called an “assault on unions” during a budget battle in Wisconsin — remarks showing his solidarity with labor protests outside the state capitol. Obama also has urged Congress to rewrite the 2002 No Child Left Behind law, which in theory could win him support from teachers. Educators often complain about the Bush-era law’s emphasis on standardized testing.

But many teachers remain leery of Obama’s school reform policies. With encouragement from the president’s signature Race to the Top initiative, at least a dozen states, including Maryland, are developing teacher evaluations that include student test scores as a factor.

To be sure, unions are not entirely opposed to linking test scores to evaluations. The AFT has been open to such ideas, and NEA leaders have signaled their willingness to discuss the issue.

Stern, a frequent substitute teacher in Montgomery, said she plans to vote for Obama because she doesn’t see a better alternative. But that might be all she does, even if her union endorses the president.

“I’m a very strong supporter of the NEA,” she said, “so it’s going to be very hard for me to not give everything that I have. But somehow I just don’t have the enthusiasm.”