Liz Cheney and Adam Kinzinger Say Secret Service Agents Won’t Testify About Jan. 6 Under Oath

"I didn't want to participate in a coup against the United States."

Well ish, their mission to protect the President outright, and one can argue that they can ignore the presidents orders if said orders involve putting him in clear and present danger. Also, they probably get the same leeway as other government service branches in that they can not be forced to follow illegal orders. The president wanting to go out to participate in a coup and ordering you to do so would fall in to all sorts of categories where "orders" could be ignored.

https://www.secretservice.gov/about/faq/general

"Today, the Secret Service's mission is two-fold: protection of the president, vice president and others; and investigations into crimes against the financial infrastructure of the United States."

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