This is some Constitutional background on the ‘Advice and Consent’ role of the Senate.
This information is from this website: www.senate.gov/artandhistory/history/common/briefing/Nominations.htm
“The United States Constitution provides that the president “shall nominate, and by and with the Advice and Consent of the Senate, shall appoint Ambassadors, other public Ministers and Consuls, Judges of the Supreme Court, and all other Officers of the United States, whose Appointments are not herein otherwise provided for…” (Article II, section 2). This provision, like many others in the Constitution, was born of compromise, and, over the more than two centuries since its adoption, has inspired widely varying interpretations.
The president nominates all federal judges in the judicial branch and specified officers in cabinet-level departments, independent agencies, the military services, the Foreign Service and uniformed civilian services, as well as U.S. attorneys and U.S. marshals. In recent years, more than 300 positions in 14 cabinet agencies and more than 100 positions in independent and other agencies have been subject to presidential appointment. Approximately 4,000 civilian and 65,000 military nominations are submitted to the Senate during each two-year session of Congress. The vast majority are routinely confirmed, while a very small but sometimes highly visible number fail to receive action.
The importance of the position, the qualifications of the nominee, and the prevailing political climate influence the character of the Senate’s response to each nomination. Views of the Senate’s ‘proper role’ range from a narrow construction that the Senate is obligated to confirm unless the nominee is manifestly lacking in character and competence, to a broad interpretation that accords the Senate power to reject for any reason a majority of its members deems appropriate. Just as the president is not required to explain why he selected a particular nominee, neither is the Senate obligated to give reasons for rejecting a nominee.”