"Article 1, Section 6, Clause 2 of the United States Constitution
No Senator or Representative shall, during the Time for which he was elected, be appointed to any civil Office under the Authority of the United States, which shall have been created, or the Emoluments whereof shall have been increased during such time; and no Person holding any Office under the United States, shall be a Member of either House during his Continuance in Office."
The above is referred to as the Ineligibility Clause (or Emoluments Clause or the Incompatibility Clause, or the Sinecure Clause). It is basic US Constitution knowledge that the separation of powers is the bedrock of the Constitution.
Ineligibility Clause Law and Legal Definition | USLegal, Inc.
"Ineligibility Clause Law and Legal Definition
The ineligibility clause in the U.S. constitution puts a limitation on the employment of members of Congress, and the employees of the executive branch, from serving or holding other office of Congress. This clause expressly bars the senator or representatives from being appointed to any civil office under the authority of the U.S.
The purpose of the clause is twofold:
1. to protect separation of powers;
2. to prevent Congress from conspiring to create offices or increase federal officials' salaries with the expectation that members of Congress would later be appointed to these posts."