<P> The U.S. Constitution also provides for appointment of one of the elected Senators to serve as President pro tempore . This Senator presides when the vice president was absent from the body . The President pro tempore is selected by the body specifically for the role of presiding in the absence of (as the meaning of pro tempore, literally "for the time being") the actual presiding officer . By tradition, the title of President pro tempore has come to be given more - or-less automatically to the most senior senator of the majority party . In actual practice in the modern Senate, the President pro tempore also does not often serve in the role (though it is his or her constitutional right to do so), instead, as governed by the aforementioned Rule I, he or she frequently designates a junior senator to perform the function . </P> <P> When the Senate hears the impeachment trials of the President of the United States, by procedure established in the U.S Constitution, the Chief Justice of the United States is designated as the presiding officer . </P> <P> The Constitution provides for two officers to preside over the Senate . Article One, Section 3, Clause 4 designates the Vice President of the United States as the President of the Senate . In this capacity, the vice president was expected to preside at regular sessions of the Senate, casting votes only to break ties . From John Adams in 1789 to Richard Nixon in the 1950s, presiding over the Senate was the chief function of vice presidents, who had an office in the Capitol, received their staff support and office expenses through the legislative appropriations, and rarely were invited to participate in cabinet meetings or other executive activities . In 1961, Vice President Lyndon B. Johnson changed the vice presidency by moving his chief office from the Capitol to the White House, by directing his attention to executive functions, and by attending Senate sessions only at critical times when his vote, or ruling from the chair, might be necessary . Vice presidents since Johnson's time have followed his example . </P> <P> Next, Article One, Section 3, Clause 5 provides that in the absence of the vice president the Senate could choose a president pro tempore to temporarily preside and perform the duties of the chair . Since vice presidents presided routinely in the 18th and 19th centuries, the Senate thought it necessary to choose a president pro tempore only for the limited periods when the vice president might be ill or otherwise absent . As a result, the Senate frequently elected several presidents pro tempore during a single session . </P>

When does the presiding officer of the senate vote