A Koch brothers advocacy group contends California Attorney General Kamala Harris has no business demanding the names and addresses of its donors, who might face “grotesque threats” if identified.

Americans for Prosperity Foundation, co-founded by billionaires Charles and David Koch, promotes limited government and free markets and conservative causes. Compelling disclosure of the nonprofit organization’s contributors may put them at risk of being targeted by the group’s opponents and intimidate would-be donors, the foundation is set to argue Tuesday in federal court in Los Angeles.

“Grotesque threats have been leveled against known associates of the foundation, ranging from threats to kill or maim, to threats to firebomb buildings,” the organization said in its request to block California from seeking the information while it fights the state in court.

The political network overseen by the Kochs aims to raise almost $1 billion in the run-up to the 2016 U.S. presidential election. The fundraising goal of $889 million was announced on Jan. 26 at a Koch-organized summit of 450 wealthy donors and small-government activists in Palm Springs, California.

The brothers are ranked on the Bloomberg Billionaires Index as the world’s fifth- and sixth-richest people with a combined net worth of $100 billion.

Senate Campaign

Harris, a Democrat who won re-election November, is running in 2016 for the U.S. Senate seat being vacated by Barbara Boxer.

In 2013 Harris helped secure a $1 million settlement with two Arizona-based political groups that California election regulators said had ties to the brothers. The agreement resolved claims the groups violated California campaign finance laws by concealing the origin of $15 million in donations to conservative causes in the 2012 election.

The Americans for Prosperity Foundation sued Harris in December, accusing her of violating free-speech rights that protect anonymous donations. The attorney general, “out of the blue,” started asking in 2013 that it provide the federal tax form identifying its donors, according to Americans for Prosperity.

An auditor for the attorney general’s office said that the confidential donor information is used to investigate potential fraudulent conduct, such as donors using a charity to fund their business or pass money to family members, according to a Jan. 21 filing in federal court in Los Angeles.