Wisconsin Gov. Scott Walker’s victory in the recall election Tuesday amounted to a significant defeat for the nation’s labor unions, who had mounted one of their most aggressive grass-roots campaigns ever to defeat the Republican governor.

As one of his first acts after taking office two years ago, Walker had targeted the unions representing state workers, moving to curb their collective bargaining rights.

The failure of the union effort to oust him Tuesday sent reverberations across the labor movement and the Democratic Party, signaling that one of President Obama’s most powerful constituencies is politically vulnerable and may not be able to help him as much as expected in this year’s election, either in Wisconsin or across the country.

Allied labor groups rallied more than 50,000 volunteers who knocked on 1.4 million doors and placed 1.8 million calls, according to figures from the American Federation of State, County and Municipal Employees.

The AFL-CIO deployed its targeting and technical teams to rally opposition against Walker.

The weeks-long standoff at the state Capitol last year, which drew tens of thousands of union supporters and national attention, had invigorated the movement, too.

But even after the groups allied against Walker spent $18 million, it was insufficient to match the sitting governor, whose campaign and supporters spent $47 million, according to figures from the Wisconsin Democracy Campaign, a nonpartisan group that tracks political spending.

Union leaders are rejecting the idea that the outcome reflected any growing antipathy with labor, or the diminished presence of unions.

The campaign “showed our ability to put boots on the ground,” said Gerry McIntee, head of AFSCME.

The unions had some success. The share of people from union households who turned out to vote rose from 26 percent in 2010 to 33 percent on Tuesday. Voters also narrowly ousted a Republican from the state Senate, shifting the balance of power there.

Moreover, in Ohio, after Republican Gov. John Kasich approved a similar law curtailing collective bargaining for state workers, voters there in November overwhelmingly overturned it.

“This was a defeat and a serious one for unions,” said Harley Shaiken, a University of California Berkeley professor who studies unionism. “But it doesn’t say they’re a paper tiger. It says they’re vulnerable.”

Walker and his supporters used much of their financial advantage on buying television advertising time, inundating the airwaves with messages arguing that his moves against the unions were necessary. “The reforms are working,” one of the ads broadcast across the state told viewers.

They also touted figures that showed that the state budget was facing a deficit of as much as $3.5 billion, amounting to a crisis that made reforms critical.

“This was a referendum on collective bargaining rights, and the unions lost,” said Luke Hilgemann, director of Americans for Prosperity Wisconsin, an issues advocacy group that spent more than $5 million in the campaign. “The taxpayers of Wisconsin made it very clear that they did not support lavish pay and benefits for government workers.”

The setback in Wisconsin pointed anew to a strategy favored by many Republicans to weaken the unions as a way of permanently weakening the Democratic Party.

Union membership has continued to decline over the years. After a decades-long slide, unions represent less than 7 percent of the private-sector workforce. Public-employee unions have significantly higher percentages of membership.

About 37 percent of the government workforce is composed of union members.

The reforms that Walker and many other Republican governors (and some Democratic governors) have sought are mostly on issues of pay and benefits. But efforts to chip away at collective bargaining rights can deal a significant blow to the unions’ treasuries and therefore their organizing strength.

Steve Rosenthal, a Democratic strategist with strong ties to labor, said the outcome was “a big loss” for the unions.

But he argued that they had no choice but to fight after Walker took away most collective bargaining rights for public employee unions. “Having been drawn into that fight, they waged an incredible campaign,” he said.

Rosenthal said he could see some potentially beneficial results from the recall battle. He predicted that other governors would not want to go through what Walker has experienced in the past 18 months and as a result will tread more lightly in trying to roll back public employee benefits and workplace rights.

AFL-CIO President Richard Trumka told reporters Wednesday that he regretted that the campaign was not waged more directly on the issue of collective bargaining rights.

“This particular election … wasn’t about collective bargaining the last month,” he said. “I wish it had been. But it wasn’t.”

Others critiquing labor’s performance argued that the problem with labor’s approach in Wisconsin and in some other campaigns is that it is too focused on a narrow, labor-oriented message instead of one that can appeal not only to union members but also to persuadable swing voters.

Tuesday’s results confirmed the worst fears of those Democrats who had privately argued against trying to oust Walker in a recall election. Obama campaign officials were among those cautioning against the recall, as were others associated with the labor movement.