WASHINGTON ― A Republican Party super PAC funding Islamophobic ads against Jon Ossoff, the Democratic House candidate in the special election in Georgia’s 6th Congressional District, is funded by undisclosed dark money and big corporations with lobbying operations in Washington.

Congressional Leadership Fund raised $4.5 million in the first three months of 2017, according to new disclosures. The super PAC, which is closely linked to Republican Party leadership, has already spent $2.4 million against Ossoff in the Georgia special election to fill the seat vacated by Health and Human Services Secretary Tom Price.

Ossoff is running in a crowded field in a district that swung heavily toward Democrats in the 2016 presidential race. The special election has been targeted by Democratic Party activists as a key test of the progressive surge sparked by the election of President Donald Trump. The first vote happens on April 18. If no candidate receives 50 percent or higher, the top two finishers will compete in a June 20 runoff election.

Republicans are trying to hold Ossoff under the 50-percent threshold by dumping big money into the race.

Congressional Leadership Fund, which is closely connected to House Republican leadership, has run a series of controversial advertisements against Ossoff in the past month. In one, the super PAC used old footage of the 30-year-old candidate dressed as Han Solo for Halloween when he was in college. Another says Ossoff has a “radical agenda” and flashes images of anarchists smashing windows. The more recent advertisements claim Ossoff is connected to the late terrorist mastermind Osama bin Laden because his company sold documentary films it produced to the Qatari-funded television network Al Jazeera.

Ossoff’s campaign manager, Keenan Pontoni, has called the ads attempting to link the candidate to bin Laden, “truly shameful.”

The biggest funder of these attacks is mysterious.

The nonprofit American Action Network contributed $3.6 million to Congressional Leadership Fund. But because American Action Network does not disclose donors, the source of the money isn’t publicly known.

American Action Network is the nonprofit affiliate of Congressional Leadership Fund. Its senior leadership features numerous lobbyists working to influence Congress for corporate and foreign clients.

American Action Network Chairman Norm Coleman, a former Republican senator from Minnesota, lobbied for corporate clients that included foreign companies in the airline industry: Airbus, the French manufacturer; Emirates, the state-funded airline of the United Arab Emirates; and Aeromexico, the Mexican airline. Coleman also is a registered foreign agent lobbying on behalf of Saudi Arabia.

American Action Network’s leadership also includes big-name corporate lobbyists Vin Weber (R-Minn.) and Tom Reynolds (R-N.Y.), both former members of Congress, and Luis Fortuno, a former Puerto Rico delegate to Congress. Barry Jackson, who was chief of staff to former House Speaker John Boehner (R-Ohio), is another top-shelf lobbyist working as a strategic adviser for the dark-money group.

Some contributors to Congressional Leadership Fund’s attacks on Ossoff are known. The largest known donor is the petroleum giant Chevron Corp., which gave $250,000 to the super PAC in March.

GEO Group Holdings, the nation’s second-largest for-profit prison company, chipped in $100,000. The private prison industry faced prospects of losing big federal government contracts under President Barack Obama and Democratic presidential candidate Hillary Clinton. GEO Group spent hundreds of thousands of dollars on super PACs to support Trump and congressional Republicans. In February, Attorney General Jeff Sessions scrapped the Obama administration’s move to phase out government use of for-profit prisons.

Congressional Leadership Fund also recently disclosed who funded its Trump inauguration party in Washington in January. Big companies lobbying Congress made up the majority of these donors. Exelon, the nuclear power utility, donated $50,000. MillerCoors, the beer giant; Anthem, a major health insurer; Southern Company, one of the largest U.S. utilities; Biotechnology Industry Organization, the lobbying arm of the biotech industry; MetLife; AT&T; and Microsoft each donated $25,000.