WASHINGTON – Prominent labor leaders, frustrated that Democrats in Washington aren’t aggressively pursuing the union agenda, are threatening to limit their campaign support for Democrats, an act that would hamper the party’s bid to regain control of the House next year and keep a majority in the Senate.

AFL-CIO President Richard Trumka’s threat of a pullback Friday was the latest warning to a party that has long relied on labor’s cash and grass-roots support. If it makes good on its threat, labor probably would spend more time and money combating union-busting efforts by state officials.

“We will change the way we spend, the way we do things and the way we function that creates power for workers,” Trumka said.

In a speech at the National Press Club, Trumka called for “an independent labor movement” and said unions were not responsible for building the power of any political party, but for improving the lives of working families. He promised that unions would spend the summer holding leaders in Congress and the states accountable.

If labor makes itself truly independent of the Democratic Party, it would mark a major shift in a long-standing political relationship.

“It doesn’t matter if candidates and parties are controlling the wrecking ball or simply standing aside to let it happen,” Trumka said. “The outcome is the same either way. If leaders aren’t blocking the wrecking ball and advancing working families’ interests, then working people will not support them.”

The AFL-CIO’s executive council is considering a plan that could spend less on congressional races and more on fighting state battles like those in Wisconsin and Ohio, where lawmakers want to weaken collective bargaining rights and reduce union clout.

But Trumka made clear that the federation had no plan to follow the lead of the nation’s largest firefighters union, which announced last month that it would halt all political donations to members of Congress because they are not fighting hard enough for union rights.

The move has won praise in many corners of the labor movement, where union activists have openly grumbled about House and Senate Democrats being too quiet while unions are getting pummeled in dozens of states.

“We’ve spent money where we have friends, and we will continue to do that,” he said.

Leon Fink, a labor historian at the University of Illinois at Chicago, said unions are tired of being taken for granted and discouraged that their influence with moderate and conservative Democrats has been limited.

“Spending a lot of money electing conservative Democrats in marginal districts had no legislative payoff for unions,” Fink said. “They don’t seem to have the capacity to impose their will on the party.”

Unions have been disappointed that Congress has not passed a more ambitious stimulus plan to create jobs, that health care reform didn’t go far enough and that Democrats — when they held a majority in Congress — couldn’t muster enough votes to pass a bill that would make it easier to organize unions.

The AFL-CIO spent more than $50 million to support Democrats in last year’s midterm elections, much of it in critical get-out-the vote efforts in dozens of key races. But a growing number of union leaders remain frustrated at what their money has bought. Some activists want to reallocate resources permanently so that more is spent bolstering grass roots support in the states.

Unions have threatened to pull support from Democrats before, only to come back as election time draws closer when they realize there are few political alternatives.

Asked how seriously Democrats should take the threat, Trumka pointed to former Arkansas Sen. Blanche Lincoln. Unions spent about $10 million last year trying to unseat Lincoln in the Democratic primary because she refused to support a broader health reform package and a bill that would make it easier for workers to form unions. Lincoln beat back the challenge, but lost in the general election.

Yet unions continued to offer support to other Democrats in the 2010 election who also wavered on the health overhaul, as some leaders feared the consequences of a GOP majority would be even worse.

It remains unclear how far the trend on unions trimming back political donations might spread. The politically powerful Service Employees International Union does not intend to reduce its role in federal races, SEIU political director Brandon Davis said.

Guy Cecil, executive director of the Democratic Senatorial Campaign Committee, said organized labor is not just an important part of the Democratic Party, but is “critical to rebuilding our entire economy.”

“We are working closely with labor at every level to build strong campaigns and deliver results for working families,” Cecil said.

Jennifer Crider, spokeswoman for the Democratic Congressional Campaign Committee, said “labor’s fight is our fight and we’re proud to partner with them.”

Trumka saved his harshest criticisms for Republicans in Congress and dozens of state legislatures for passing budgets that slash pensions and curb bargaining rights of union members while giving tax cuts to “the powerful and well-connected.”

“The final outrage of these budgets is hidden in the fine print,” Trumka said. “In state after state, and here in Washington, these so-called fiscal hawks are actually doing almost nothing to cut the deficit.”

He said these budget deals are sending a message that “sacrifice is for the weak.”

“Powerful political forces are seeking to silence working people — to drive us out of the national conversation,” Trumka said.

Trumka and other union leaders have said they expect the moves in some states to curb union rights will create a backlash that will help organized labor grow stronger. Unions are already spending millions to help recall campaigns in Wisconsin and Ohio. They are hoping the momentum of those recalls can be sustained through the 2012 elections.