A Supreme Court decision Monday involving the right of public employee unions to collect dues threatens one of the Democratic Party’s major treasure chests.

The Service Employees International Union (SEIU), which was on the losing end of the court’s 5-4 decision, has contributed more than $275 million since 1989 to campaigns at the federal and state level. That’s according to data that Sunlight’s Influence Explorer compiled from the National Institute on Money in State Politics and the Center for Responsive Politics.

The top beneficiary of the union’s largesse: Illinois Gov. Pat Quinn, the defendant named in the case that the Supreme Court decided Monday. Quinn, who is up for re-election this year, collected nearly $4.5 million from the union, nearly twice as much as he collected from his second biggest donor.

SEIU’s political activity, and whether employees who don’t agree with it should have to underwrite it, was at issue in the majority opinion: “If we accepted Illinois’ argument, we would approve an unprecedented violation of the bedrock principle that, except perhaps in the rarest of circumstances, no person in this country may be compelled to subsidize speech by a third party that he or she does not wish to support,” wrote Justice Samuel Alito.

The case, Harris v. Quinn, was brought by Pamela Harris, the mother of a disabled child in Illinois. In order to get state subsidies to act as her son’s caregiver, she had to join the SEIU, the state’s exclusive recognized bargaining unit for home-care providers. In 2010, Harris and other “personal assistants,” as they are known in the Illinois program, sued Gov. Pat Quinn claiming the forced union dues violated their First and 14th Amendment protections.

Another top recipient of SEIU dollars: past Illinois Gov. Rod Blagojevich, who signed the executive order that gave the SEIU to right to represent Illinois personal assistants in 2003. Blagojevich received about $2 million from the labor group, according to Influence Explorer.

In Monday’s decision, the high court’s majority decided that workers like the Illinois caregivers — who are not full-time state employees — don’t have to pay union dues. But the justices stopped short of outlawing so-called “agency shops,” in which all employees are required to ante up to the collective bargaining unit, whether they want to or not.

Given Alito’s sweeping condemnations in the majority opinion, however, he decision could open the door to further challenges that could hamstring the ability of public-employee unions to raise money to participate in the political process.

But the SEIU has been formidable about protecting its ability to fill its political war chest. In 2005, the SEIU also spent about $11.7 million to defeat Proposition 75, a California ballot initiative aimed to require public-employee unions to get “annual, written consent from a government employee in order to charge and use that employee’s dues or fees for political purposes,” according to an analysis prepared by the non-partisan Legislative Analyst’s Office.

The SEIU’s political giving pales in comparison to other public employee unions. While that union contributed around $18.5 to state and federal races during the 2011-2012 cycle, according to Sunlight’s influence explorer, its influence trailed behind three other unions who combined contributed about $90 million nationwide. These totals don’t include local union affiliates, some of which make substantial contributions.

As the battle over “right to work” legislation at the heart of this case heats up, the spending between unions and the well-heeled conservative groups fighting to end collective bargaining is likely to catch flame as well.

According to the National Conference of State Legislatures, a non-partisan collective of state governing bodies, 24 states have laws on the books that allow  public employees to opt out of union membership. While most of these laws were passed in the years immediately following World War II, a sudden resurgence in so-called “right to work” legislation came in 2012, when both Indiana and Michigan enacted new laws governing public employee unions operating in those states. “Right to work” legislation was introduced in 23 states last year, with Tennessee passing legislation, according to the NCSL.

At least seven such bills providing for a “right to work” have been introduced in state legislatures this year, according to data from the Sunlight Foundation’s Open States project.

The latest battleground pitting unions against groups like Koch-backed groups like Americans for Prosperity and the American Legislative Exchange Council is taking place in Missouri, where a possible ballot measure for this August would decide the fate of collective bargaining in that state.

While not as transparent with their giving, conservative groups have shown no hesitation to spend big to hobble unions in races around the country. In 2012, a group with links to the Koch brothers’ network spent several million dollars trying to pass a ballot that would outlaw using membership dues for political purposes in California, according to the Los Angeles Times. In Wisconsin’s bitter fight between its scandal-embroiled Gov. Scott Walker and public employee unions over wages and bargaining rights, money from the Koch’s financed not only Walker’s anti-recall campaign but also bankrolled groups like Americans for Prosperity, key Walker allies, according to the New York Times.

One such Koch-allied group wasted no time in celebrating today’s Supreme Court decision. The Illinois Policy Institute, a listed “partner organization” of the Charles Koch Institute, published a blog post entitled “Victory: Harris v. Quinn” shortly following the release of the ruling.

“For more than a decade, government unions have been forcing people who are not state workers – moms and dads caring for children with developmental disabilities, home day-care providers for low-income children and others – to pay dues to a union as a condition of receiving help from their state governments,” the institute’s Director of Labor Paul Kersey wrote. “Fortunately, today the U.S. Supreme Court has affirmed that plaintiff Pam Harris won’t have to jeopardize and limit her son’s care by being forced to join a union she does not want, agree with or support.”