First, the Constitution’s separation of powers provisions often provide for one part of government to propose an action subject to the approval of another part. The president negotiates treaties and the Senate consents or does not consent to them. One chamber of Congress passes bills and the other chamber passes or does not pass them. The president proposes laws and Congress enacts or does not enact them. Congress proposes constitutional amendments and state legislatures ratify or do not ratify them. The Constitution is not read in any of these situations to impose a duty on the second entity to act formally on the proposal. If the second entity fails to approve, for whatever reason and in whatever manner, the measure does not take effect.
The one exception is presentment. Article I, Section 7 says that when Congress passes a bill, the president ordinarily must veto the bill within 10 days—and give reasons for doing so—or the bill becomes law. The framers knew how to require formal action if they wanted to; they just chose not to require it in the appointments clause (and elsewhere).
Second, by Article I, Section 5, the Senate has power to “determine the Rules of its Proceedings.” As a result, the Constitution expressly lets the Senate decide how to respond to presidential nominations. The Senate could, for example, adopt a rule that it will promptly hold votes on all presidential nominations. But that is the Senate’s decision to make, and for better or worse it’s adopted a different approach, which is that it can decide not to have formal proceedings and instead withhold its consent through an informal process.
Some critics say the Senate is refusing to “consider” Garland’s nomination, but that’s mistaken: Senators are aware of the nomination; they have thought about it and decided that formal action should wait until after the presidential election. The critics’ claim—that it doesn’t count as “considering” unless the Senate acts formally—is exactly contrary to Article I, Section 5, which says the Senate decides on its rules of procedure. In this case, the procedure that’s been adopted is for the majority leader and the chair of the Senate Judiciary Committee to convey the Senate majority’s decision not to consent to the appointment (at least until after the election).
Third, the Senate’s longstanding practice, at least in modern times, is often not to act formally on nominees. This practice has mostly involved nominations to lower courts and executive branch offices (as with the Senate’s failure to vote on numerous judicial nominations by President George W. Bush). But the appointments clause applies to all nominations equally; if the clause imposed any duty on the Senate, the duty would encompass all nominations, not just Supreme Court nominations. Moreover, the Senate has in modern times assumed that Supreme Court nominations can be filibustered (which has the effect of denying a formal vote). Indeed, the Senate did filibuster Lyndon Johnson’s nomination of Abe Fortas to be chief justice, and many senators (including then-Senators Barack Obama and John Kerry) joined an unsuccessful filibuster against Samuel Alito. The Senate’s practice, under both Democrats and Republicans, shows that it thinks the appointments clause does not impose a duty to take formal action.