David Koch, executive vice president of Koch Industries Inc., smiles as Mark Carney, governor of the Bank of England, not seen, speaks to the Economic Club of New York in New York, U.S., on Monday, Dec. 9, 2013. Carney said Britain's recovery will need to be sustained for a while before it is strong enough to withstand higher interest rates. Photographer: Jin Lee/Bloomberg via Getty Images

WASHINGTON -- Not a week goes by these days without Senate Majority Leader Harry Reid (D-Nev.) denouncing the billionaire Koch brothers as "un-American" and "power-hungry tycoons" for funding a massive independent political machine working to defeat the Democratic Senate majority.

Republicans have called Reid's comments an attack on private citizens and free speech and an attempt to stifle conservative voices. Sen. Pat Roberts (R), who represents the Kochs' home state of Kansas, labeled Reid's remarks as "bizarre" and, at a April 30 committee hearing, said, "The First Amendment doesn’t allow us to silence those who oppose us."

Campaigning against wealthy and powerful interests, however, has a long history in American politics. It's not unique to Reid or to the Democratic Party.

Over the years, congressional Republicans have similarly used their positions in power to denounce Democratic supporters, including labor unions, trial lawyers and, of course, billionaire financier George Soros.

The past criticism of Democratic donors may not have matched the verbal ferocity of Reid's current denunciations of the Kochs. But in the case of attacks on labor unions and their political activity, Republican lawmakers have gone even further by initiating congressional investigations, filing election complaints and seeking legislation to gut union membership and dues -- in other words, a direct legislative assault on unions' very existence.

Following the 1994 Republican takeover of Congress, organized labor became more aggressive politically under the new leadership of AFL-CIO President John Sweeney and political director Steve Rosenthal. In 1996, the AFL-CIO spent $35 million on its political efforts, including voter registration drives and issue-based television ads attacking vulnerable Republican incumbents.

Rep. John Boehner (R-Ohio), then one of House Speaker Newt Gingrich's lieutenants, said that labor had "declared war on the new Republican Congress and we have no choice but to respond."

That response included launching committee investigations into union political activity, pressuring television and radio stations to pull allegedly inaccurate ads, filing a complaint with the Federal Election Commission claiming election law violations by unions, and introducing legislation to prevent unions from spending money on politics and to limit how unions receive funds from their members.

A special investigation into union political spending, dubbed the American Worker Project, was chaired by Rep. Peter Hoekstra (R-Mich.) and organized in coordination with outside groups -- including the Koch brothers' Citizens for a Sound Economy, a precursor organization to their Americans for Prosperity. An article from National Journal's CongressDaily dated Feb. 9, 1998, raised questions about the role CSE played in setting up some of the committee's events outside Washington, though an official with the group would not say how much money it spent on the effort.

This was also the beginning of congressional Republican support for the proposed Paycheck Protection Act. The legislation -- pushed at the federal and state levels -- would require unions to obtain written authorization from individual dues-paying members to spend their money on political activity and would almost certainly lead to a drop in available funds. Currently, union members must opt out if they do not want their dues to go to political activity.

In the 1998 elections, Republicans attacked Democrats like Rep. Leonard Boswell (Iowa) as hailing from the "Labor-Democrat Party" -- a formulation similar to Reid's current criticism of the Koch-Republican agenda.

GOP attacks on unions and their contributions to Democrats continued. During debate over the creation of the Department of Homeland Security in 2002, Republicans insisted on adding a provision allowing President George W. Bush to waive established personnel rules for the new department, thereby undercutting the public employees union. When Democrats opposed this measure, Republicans said they had been bought by the unions and circulated labor's fundraising numbers to reporters.

"I question their judgment putting special interest politics above the common good," Rep. Lindsey Graham (R-S.C.), then running for a Senate seat, said about Senate Democrats and his Democratic opponent Alex Sanders.

More recently, Republicans in state capitols across the country have sought to crush public and private unions to both aid business and empty Democratic Party coffers.

Wisconsin state Sen. Scott Fitzgerald (R) made this very clear in a 2011 Fox News interview about Republican Gov. Scott Walker's effort to limit collective bargaining rights for public employees. "If we win this battle, and the money is not there under the auspices of the unions, certainly what you're going to find is President Obama is going to have a much difficult, much more difficult time getting elected and winning the state of Wisconsin,” Fitzgerald said.

By 2012, the Republican Party platform supported a wide range of anti-union laws and policy changes that would gut membership and make it harder to raise dues.

After years of affirming support for states enacting so-called right-to-work laws, the 2012 platform backed a national law to prohibit unions from collecting mandatory dues or fees from employees in unionized workplaces. This came as Indiana and Michigan, under Republican governors, became the first two states outside the South and Mountain West to pass right-to-work laws.

The 2012 GOP platform also supported legislation to prevent the government from collecting dues for public employee unions through an automatic deduction from members' paychecks.

Recently, Democrats have tried to curtail the influence of billionaires by proposing small-bore campaign finance fixes, like mandatory donor disclosure, or pie-in-the-sky constitutional amendments to limit the amount of money in politics. While the Democrats are likely most concerned about the Kochs, their reforms would limit wealthy interests backing both parties.

Republicans, however, remain focused on breaking the unions and cutting off the flow of their funds to the Democratic Party. At the 2014 Conservative Political Action Conference, anti-tax activist Grover Norquist declared that unions are "not dead yet -- they're in decline. They raise maybe $7 billion a year in dues. Imagine how much they spend of that on politics. They are the largest political player in American politics and will be for some time. What can we do about it?"