https://osc.gov/pages/hatchact.aspx
The Hatch Act, a federal law passed in 1939, limits certain political activities of federal employees, as well as some state, D.C., and local government employees who work in connection with federally funded programs. The law’s purposes are to ensure that federal programs are administered in a nonpartisan fashion, to protect federal employees from political coercion in the workplace, and to ensure that federal employees are advanced based on merit and not based on political affiliation.
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The Hatch Act provision most commonly invoked in discussions of Comey's letter is 5 U.S.C. § 7323(a)(1), which prohibits a government employee from "us[ing] his official authority or influence for the purpose of interfering with or affecting the result of an election."
The key text is the emphasized phrase -- which conditions a violation of the statute on whether the employee's purpose was to interfere with or affect the result of an election. Thus, the Hatch Act does not focus on the effect of the employee's conduct, but the intent. To that end, if Comey did not intend to interfere with or affect the upcoming election through his letter to Congress, then he did not violate the letter of the Hatch Act.
http://www.cnn.com/2016/10/31/politics/what-is-the-hatch-act/
So, I don't know if you can prove he intended to interfere with the election.