Although the field of Republican presidential contenders is still in flux, the National Education Association (NEA) decided in early July to endorse President Obama’s 2012 re-election bid. The move by the nation’s biggest teachers union was entirely expected. The NEA—the fifth biggest giver to political campaigns in the past 20 years, according to the Center for Responsive Politics—supports almost no one but Democrats for federal office, having given just 5% of its campaign contributions to Republicans since 1990.

Don’t expect anything different from the nation’s other biggest unions, private or public, including the dozen unions on the Center’s list of the top 20 contributors to federal elections. Those unions, ranging from the American Federation of State, County and Municipal Employees to the Service Employees International Union to the Teamsters, overwhelmingly support Democrats. They have given only 3% of the $384 million they’ve contributed to candidates for federal office over the past 20 years to Republicans. Union advocates argue that this is because the Democratic Party looks out for the interests of union members. But a significant plurality of their members doesn’t always agree.

Exit polls estimate that 59% of union members voted for President Obama in 2008, a particularly strong Democratic year in which the president garnered 53% of the overall vote. In 2010, union members swung somewhat the other way, an ABC News/Washington Post poll found, with 54% of union members voting for Democratic congressional candidates and 42% for Republicans.

While there are no polls specifically on NEA members, data from the General Social Survey (GSS) suggest that grade school teachers in general have voted Democratic only somewhat more often than they have Republican in presidential elections dating back to 1972. Teachers have favored Democrats by 50% to 44% for the GOP (with the rest voting for independent candidates), according to an analysis of the massive GSS database by the blog The Audacious Epigone.

In recent years, as private-sector union membership has declined while public-sector union ranks have grown, the movement’s leadership has focused not just on traditional labor issues like raising the minimum wage, but also advocating consistently for bigger government and more public spending.

Unions, for instance, were behind more than two dozen lobbying campaigns and initiative drives at the state level between 2007 and 2010 to raise taxes or boost spending. They spent millions of dollars in 2010 on Oregon ballot measures 66 and 67, which raised business and income taxes in the state, and some $3.5 million in Washington state in support of a failed ballot initiative to institute an income tax.

Labor wasn’t so uniformly behind tax increases when the movement was dominated by private labor groups. In California in 1978, many private-sector union members backed Proposition 13, the initiative that capped property taxes, sparking discord within the AFL-CIO because of the heavy opposition of public union leaders to the tax cap. Even rank and file government union members weren’t uniformly opposed. Polls showed that 40% of them voted for Prop 13, according to a 1978 Washington Post story on the turmoil that the initiative campaign caused within the labor movement.

Today’s union leaders have also increasingly directed members’ dues to causes that are only marginally associated with labor issues. When the administration of George W. Bush began requiring unions to file more extensive disclosure forms detailing how they spent dues, the documents revealed, for instance, that the NEA was using dues money to fund groups like the Gay and Lesbian Alliance Against Defamation, Jesse Jackson’s Rainbow Push Coalition, and Amnesty International.

Union affiliates in some states are heavily involved in spending members’ money on social causes that are sometimes contrary to members’ own political leanings. For example, California unions donated more than $2 million—including $1 million from the California Teachers Association—to a campaign in 2008 to defeat Proposition 8, the successful state initiative that defined marriage as between a man and a woman. A CNN exit poll found that 56% of voters in union households supported the initiative despite the opposition of the union leadership.

“The California Teachers Association does not poll its members on how it spends its political money,” Larry Sand, a union critic and 28-year veteran of the state’s school system, observes. Instead, union leaders vigorously oppose efforts to allow their members more control over how their dues are spent.

In California, for example, labor groups and their allies expended an astounding $54 million in 2005 to defeat Proposition 75, which would have required that unions seek the permission of members to spend dues money on political contributions. Among the biggest nonunion contributors to the campaign to defeat Prop 75 was the Democratic Party, which kicked in $3.275 million.

In Washington state, meanwhile, a judge ruled in 2000 that the Washington Education Association violated a state law barring the use of union dues for political purposes without employees’ permission. The union fought the case all the way to the U.S. Supreme Court before losing in 2007 and agreeing to pay nearly $1 million in fines and rebates to teachers.

The NEA’s leadership has not been enthusiastic about President Obama because his education secretary, Arne Duncan, touts policies like merit pay for teachers and the expansion of charter schools, which the union opposes. Grumble as they do, the NEA’s leadership has nowhere else to go. Sadly, neither do the 40% or so of union workers who may not vote for the president but will see a large portion of their dues spent on his re-election.

Mr. Malanga is a senior fellow at the Manhattan Institute and the author of “Shakedown: The Continuing Conspiracy Against the American Taxpayer” (Ivan R. Dee, 2010).