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According to a quick and dirty google search;

At the start of each new Congress, the entire House of Representatives and one-third of the Senate are sworn into office. This oath-taking dates to 1789, the first Congress; however, the current oath was fashioned in the 1860s, by Civil War-era members of Congress.
The current oath was enacted in 1884:
I do solemnly swear (or affirm) that I will support and defend the Constitution of the United States against all enemies, foreign and domestic; that I will bear true faith and allegiance to the same; that I take this obligation freely, without any mental reservation or purpose of evasion; and that I will well and faithfully discharge the duties of the office on which I am about to enter: So help me God.

The public swearing-in ceremony consists of Representatives raising their right hands and repeating the oath of office.

It is apparent Senator Manchin does not take his oath seriously......