POTUS should be protected by The Whistleblower Protection Act
POTUS a Federal employee? YES
POTUS reasonably believed there was evidence UKRAINE violated regulations governing their loan guarantee? YES
Whistleblower protection laws and regulations guarantee freedom of speech for workers and contractors in certain situations. Whistleblowers are protected from retaliation for disclosure of information which the employee or applicant reasonably believes evidences a violation of any law, rule, or regulation, or gross mismanagement, a gross waste of funds, an abuse of authority, or a substantial and specific danger to public health or safety.
5 U.S.C. 2302(b)(8)-(9), Pub.L. 101-12 as amended.
BLACK AND WHITE WHISTLEBLOWER RETALIATION AGAINST POTUS
further 5 USC 2301 says this:
(8)Employees should be—
(A)protected against arbitrary action, personal favoritism, or coercion for partisan political purposes, and
(B)prohibited from using their official authority or influence for the purpose of interfering with or affecting the result of an election or a nomination for election.
https://www.law.cornell.edu/uscode/text/5/2301
Whistleblower Protection Act:
https://www.law.cornell.edu/uscode/text/5/2302