U.S. ConstitutionArticle I — The Legislative BranchSubarticle 3Section 5
The Senate shall chuse their other Officers, and also a President pro tempore, in the Absence of the Vice President, or when he shall exercise the Office of President of the United States.
Use as reference
Lexinter.net | U.S. Const. art. I, § 3, cl. 5 | https://laws.lexinter.net/constitution/us/article-1/section-3/clause-5