In the midst of the contentious 2009 gubernatorial race in New Jersey, the state’s teachers union took a poll of its own members and found only a slight majority preferred the candidate the union had endorsed, Democratic incumbent Jon Corzine, over Republican challenger Chris Christie. The alarmed union, the New Jersey Education Association (NJEA), swung into action with a campaign that included phoning 100,000 of its members and urging them to vote for Mr. Corzine, according to union documents leaked last November to the Education Intelligence Agency, a watchdog web site.

Mr. Christie eventually won that election, in part because the NJEA, one of the most powerful political forces in the state, had to fight a rearguard action to keep its own membership in line. In that respect, the Jersey race may be a harbinger of the elections in November.

Unions used their considerable clout in 2006 to help Democrats gain control of Congress and again in 2008 to elect President Obama. But the union movement, which spent 96% of its money supporting Democrats in 2008, is faltering this year in its efforts to help the party retain control of Congress and win key governors’ races around the country.

Instead, organized labor— increasingly dominated by public-sector workers—is facing a backlash from taxpayers because of widespread publicity about the rich pay and benefits of some government employees. That’s made Mr. Christie’s blunt campaign talk about reining in government costs a popular approach among candidates. Even old friends of labor in the Democratic Party have made public workers a target, leaving labor with fewer allies and playing defense.

In California, where unions are as powerful as in New Jersey, Republican candidate for governor Meg Whitman is taking on the unions that weakened Gov. Arnold Schwarzenegger when he challenged them with reforms in 2005. One of the key players was the militant California Nurses Association (CNA), which represents some 87,000 health-care workers. The group followed Mr. Schwarzenegger around the state picketing his appearances, and when he struck back in speeches his attacks backfired and swung some voters to the union camp.

The CNA is trying the same tactic with Ms. Whitman, protesting at her home, infiltrating her invitation-only events, and portraying her in ads as Queen Meg, an imperial candidate out to hurt the working class. But Ms. Whitman has fought back by claiming, in an echo of the Jersey election, that the union leadership is out of touch with members. She obtained a database of union member addresses and mailed them campaign literature that said, “Don’t take the union bosses’ word for it . . . Learn for yourself where Meg Whitman stands.”

She’s started a group, Nurses for Meg Whitman, to counter the nurses union, and she says polls show many licensed nurses in the state support her. Facing an electorate that’s now more receptive to a message of cutting government spending and reining in worker costs, Ms. Whitman has remained neck-and-neck in the polls with Democrat Jerry Brown, who has the backing of all major labor organizations in the state.

The backlash against public unions has gone beyond heavily unionized states like California and New Jersey. One illustration is the finding of a July 7 national Rasmussen poll: Only 19% of Americans said that they would be willing to pay higher taxes to keep government workers from being laid off. Even in public safety, where Americans are sometimes reluctant to see cutbacks, the poll found only 34% endorsed higher taxes to preserve police and fire jobs.

The electorate may also be turning away from public unions because of their relentless campaigning for higher taxes. Mr. Christie has estimated that New Jersey’s public unions spent some $4 million throughout the spring on ads advocating higher taxes and railing against his budget. In California, the teachers union has kicked in $500,000 as part of a campaign to rescind business tax breaks to keep jobs in the state. Last year in Michigan, a coalition of unions engineered a campaign called “A Better Michigan Future” that advocated hundreds of millions in new taxes, which the state legislature rejected.

The prospect of ever-higher taxes has Democrats distancing themselves from labor. New York Democratic gubernatorial candidate Andrew Cuomo is preaching fiscal prudence and says public pensions are “out of line with economic reality.” In California, old allies of labor like Los Angeles Mayor Antonio Villaraigosa (who was once a teachers union official) are also inveighing against the cost imposed by public unions. Oregon’s Democratic Gov. Ted Kulongoski, an attorney who once represented unions, is advocating clamping down on public-sector pay and benefits to fix that state’s budget problems.

Unions are also on the defensive in the culture wars. Later this month the documentary “Waiting for Superman,” about the failings of our public schools, will debut in theaters nationwide. The film is directed by Davis Guggenheim, who earned impeccable liberal credentials as the director of the Oscar-winning “An Inconvenient Truth.” His new documentary, say reviewers who’ve seen it, places a chunk of the blame for the woes of our schools on teachers unions and in particular paints Randi Weingarten, the head of the American Federation of Teachers, as an opponent of meaningful reform.

Mr. Guggenheim’s film is likely to exacerbate growing discontent with teachers unions. In a May Rasmussen poll, only 38% of Americans said it was good that teachers belong to unions, while 62% either thought teacher unionization a bad thing or were undecided.

Labor’s clout is also suffering because of growing liberal fractiousness and disappointment that the Democratic victories of 2006 and 2008 haven’t led to more radical change. Unions spent $10 million in an unsuccessful attempt to defeat Arkansas Sen. Blanche Lincoln, whom they considered not liberal enough, in the state’s Democratic primary.

In North Carolina this spring, a coalition of leftist advocacy groups and unions led by the Service Employees International Union (SEIU), arguably the nation’s most powerful union, attempted to split from the state’s Democratic Party to form a more left-leaning third party. Although the effort failed, it has fractured the coalition that propelled President Obama to a narrow victory in that state in 2008.

Of course even with its influence waning, labor can be a powerful electoral force. The AFL-CIO, SEIU and the American Federation of State, County and Municipal Employees have announced plans to spend about $100 million on the November elections. While that’s less than 2008, when unions gave $73 million in direct contributions to candidates and spent another $80 million independently on campaigns, the money represents a formidable commitment that could be a factor in close races.

Still, what we are seeing this year may mark a historic shift in American politics. If candidates around the country can repeat Mr. Christie’s strategy of winning office by taking on public unions, we could be witnessing a change akin to what happened in the late 1970s, when tax revolts in a handful of states created a nationwide momentum that eventually elected Ronald Reagan.

The early 21st century version of tax rebellion is a head-on collision between overburdened taxpayers and public-sector unions. The many signs of union weakness suggest that after decades of expanding power, government-worker unions may have finally met their match.

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