The U.S. Code?
Whoever, directly or indirectly, promises any employment, position, compensation, contract, appointment, or other benefit, provided for or made possible in whole or in part by any Act of Congress, or any special consideration in obtaining any such benefit, to any person as consideration, favor, or reward for any political activity or for the support of or opposition to any candidate or any political party in connection with any general or special election to any political office, or in connection with any primary election or political convention or caucus held to select candidates for any political office, shall be fined under this title or imprisoned not more than one year, or both.
Let me pull out the verbiage that puts the quid pro quo of an unannounced candidate into the legal hopper:
Whoever, directly or indirectly, promises any... position... provided for... by any Act of Congress... to any person as... reward... in connection with any primary election or political convention or caucus held to select candidates for any political office
Promising a reward to a person, in the form of a Federal job, for them not to run is a violation of this particular law.
Whoever, directly or indirectly, promises any... position... provided for... by any Act of Congress... to any person as... reward for any political activity... in connection with any primary election or political convention or caucus held to select candidates for any political office
The parsing you cited as "reward... in connection with any primary election" could refer to paying people for "get out the vote" activities. That "reward" is the quid, but what is the "quo?" The "quo" is not any activity relating to an election.
I'm saying that the "quo" is "any political activity," but not a "political inactivity." You cannot criminalize not running. You can criminalize running and then taking a "reward" to stop running.
-PJ