Via Just Security:

Secretary of State Mike Pompeo is asserting that Congress is exceeding its authority and trying to bully State Department employees by requesting their testimony about alleged White House and State Department misconduct. His intransigence not only threatens to topple our constitutional system of checks and balances, but it attempts to nullify a basic right federal employees have enjoyed for over a century: the right to communicate with Congress free from intimidation, bullying and unfair harassment.

The Lloyd-La Follette Act of 1912, which granted federal employees this right, reads in relevant part:

The right of employees, individually or collectively, to petition Congress or a Member of Congress, or to furnish information to either House of Congress, or to a committee or Member thereof, may not be interfered with or denied.

As one of the first statutes enacted specifically to protect federal whistleblowers, the Lloyd-La Follette Act was passed, according to its accompanying House Report, “to protect employees against oppression and in the right of free speech and the right to consult their representatives.” This was especially pertinent in its time, as Presidents Theodore Roosevelt and William Taft had issued executive orders gagging federal employees from communicating with Congress, and punishing violators with termination.

This right applies to all State Department employees, current and former, who wish to testify before congressional committees.
[…]
Whistleblowers and the information they disclose are the life-blood of our system of constitutional checks and balances.  But their vital role means there is an equally strong imperative to silence or discredit them by those threatened by their truth-telling. They should prepare as if it were the most important test of their professional lives. Because it will be.

Read more:

A primer on federal employees' rights when it comes to communicating with Congress.

From @GovAcctProj's @mcculloughirvin & Tom Devine.https://t.co/QPGPilwxNs

— Just Security (@just_security) October 4, 2019

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