Rep. Nancy Mace's former top aide,Cassian Grant Daniel Hanlon, filed paperwork on Friday to run against his old boss in South Carolina's June congressional primary. 

According to the Federal Election Commission, Hanlon has filed to run for Congress in South Carolina's first district, which Mace has represented since 2021. Hanlon served as her chief of staff until he left Mace's office in December. Business Insider first reported that Hanlon had filed the paperwork to run. 

It's a rare thing for a former congressional aide to run against his or her old boss, but Mace's office has seen waves of departures, and Hanlon isn't the first to publicly express his displeasure with Mace. Mace's former spokesperson, Natalie Johnson, tweeted this when Hanlon and other top aides left Mace's office at the end of 2023: "You mean to tell me that the woman who's had six (seven?) communications directors since me in a two-year span has a toxic workplace? Who could've seen this coming!?"

Mace started out in Congress as somewhat of a Trump critic, speaking out against him when many in her party preferred to stay quiet, particularly after the attack on the Capitol of Jan. 6, 2021. But over time she has shifted her place within the GOP conference. 

Mace took many by surprise when she voted to oust former House Speaker Kevin McCarthy in the fall. She also endorsed former President Donald Trump over former South Carolina Gov. Nikki Haley, despite the fact that Haley campaigned for Mace when she faced a Trump-backed primary challenger. 

The Washington Post has reported that McCarthy encouraged Hanlon to run against Mace. 

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• South Carolina

Kathryn Watson

Kathryn Watson is a politics reporter for CBS News Digital based in Washington, D.C.