Some of the nation’s largest labor unions are cutting back dramatically on their financial support to the Democratic Party, saying they are highly frustrated with the failure of Democrats to put up stronger resistance to Republican proposals opposed by labor.

The unions have cited what they see as Democrats’ tepid response to Republican efforts to eliminate collective bargaining rights for public sector workers, cut Medicare funding and require voters to show identification at the polls.

“It doesn’t matter if candidates and parties are controlling the wrecking ball or simply standing aside,” said Richard Trumka, president of the AFL-CIO, in a speech Friday. “The outcome is the same either way. If leaders aren’t blocking the wrecking ball and advancing working families’ interests, working people will not support them.”

The determination of the unions, who have traditionally been among the largest campaign donors, to use money as a carrot and stick over policy matters could ultimately play a significant role in next year’s elections, seriously harming some Democrats’ chances of election.

“We never take anyone’s support for granted,” said Democratic Party spokesman Hari Sevugan. “And we are confident that when working men and women face a choice between a party . . . that wants to end the right to collectively bargain versus one that secured universal health care, expanded middle-class tax cuts and saved the American auto industry, we’ll be working with organized labor to again elect Democrats up and down the ballot next fall.”

Unions are simultaneously shifting their money and attention to focus more on political races at the state level, where several legislatures have targeted bargaining rights for state employees.

In the first quarter of this year, union political action committees sharply cut back funding for House Democrats, according to an analysis of federal disclosure reports by The Washington Post. Those contributions fell by half compared with the first quarter of 2009, from $5.8 million down to $3.1 million.

By comparison, corporate PACs cut their contributions to House Democrats by 26 percent, to a total of $7.2 million. Union contributions to Republicans decreased as well, but by just 13 percent.

The most dramatic shift was in giving by the International Union of Operating Engineers, which represents construction workers and has a large federal PAC. In the first quarter of 2009, the union gave $1.6 million to House Democrats, while the PAC this year has not made a single contribution to either party.

Officials with the engineer’s union said in a statement that high unemployment in the construction sector was its top priority and that it “wants to see Congress more urgently address this issue on a bipartisan basis and move on legislation to create jobs.”

The United Brotherhood of Carpenters and Joiners gave $350,000 to House Democrats in the first quarter of 2009 but donated only $148,000 in the first three months of this year. A spokesman for union, which left the AFL-CIO in a 2001 split of the federation, could not be reached.

The International Association of Fire Fighters announced last month that it would indefinitely halt all political giving on the federal level, citing what it said was the weak response of congressional leaders to legislative threats in the states to unions.

“I have not seen our friends in these incredible attacks against us across the country,” said Harold Schaitberger, the union’s president. “Where are our friends in Congress? Where have they been to fight back on our behalf with the same voracity and the same discipline of our enemies?”

Schaitberger also cited major disappointments at the federal level, including the deal between President Obama and Congress to extend Bush-era tax cuts for upper-income Americans and the defeat of the Employee Free Choice Act, which would have made it easier for unions to organize.

“There’s just been a pattern of disappointment and failures in advancing an agenda that helps the working middle class,” he said. “It’s a pattern that goes back years.”

It is unclear whether unions will end up backing Obama in his 2012 reelection campaign with the same enthusiasm as they did in 2008. Trumka voiced muted criticism of the president on Friday, saying he didn’t “make the honor role” for the execution of his agenda. He faulted Obama for losing a message war with Republicans over stimulus funding and pushing a free-trade agreement with Colombia.

Labor’s threats to Democrats follow a major push in last year’s midterm election, when unions spent $8 million backing a liberal challenger to former senator Blanche Lincoln (D-Ark.). The challenger, then-lieutenant governor Bill Halter, lost to Lincoln in a runoff, and a weakened Lincoln went on to lose the general election to Republican John Boozman.

Trumka trumpeted the outcome of that race in a question-and-answer period after his speech Friday. A moderator asked what was different about his latest rhetoric given that unions have threatened to withdraw support for Democrats in the past.

“Ask Blanche Lincoln,” he replied.