Actually, the Constitution states:
... he shall nominate, and by and with the advice and consent of the Senate, shall appoint ... judges of the Supreme Court, and all other officers of the United States, whose appointments are not herein otherwise provided for, and which shall be established by law:
Federal Appeals judges being an "other officer of the United States". Note that nowhere in this does it say on what basis the Senate shall make its advice or grant or withhold it's consent.
Your premise that the Senate is supposed to advise the President and grant or withhold it's consent based on whether or not the candidate is qualified is not backed up by the wording of the Constitution. There are no limits or guidelines in the Constitituion regarding the basis on which the Senate can exercise this power.
There are legitimate reasons for opposing the Senate's actions regarding this man, but none that I can see that are based on the Constitution.
Not only that, the procedures that created the Federal Circuit Courts of Appeal, and that provide for filling vacancies, is not even in the Constitution, but are in 28 USC. The Constitution recites that "[the President] ... shall appoint ... all other Officers of the United States, whose appointments are not herein provided for, and for which shall be established by Law: but the Congress may by Law vest the Appointment of such inferior Officers, as they think proper, in the President alone, in the Courts of Law, or in the Heads of Departments." (Article II, Section 2). The appointment of judges for the courts of appeal is not directly provided for in the Constitution.
Congress has the power "To constitute Tribunals inferior to the supreme Court" (Article I, Section 8). 28 USC 44(a) describes the procedure for appointing circuit judges, and happens to mirror the language of the Constitution. "The President shall appoint, by and with the advice and consent of the Senate, circuit judges for the several circuits as follows: [list of circuit courts of appeal and the number of judges in each]
Implicit in the Constitution however, is that the default mode for the existence of Congressional consent is majority, i.e. 50% plus one vote. Some decisions require more than that (e.g., two thirds vote required to expel a member, or to over-ride a Presidential veto) and some require less (recording of yeas and nays requires one fifth of members present).
Without preparing a reasoned justification, my initial impression is that the Democrats in the Senate are abusing their procedural prerogative, by effectively asserting that 60% of the members are required in order to conclude that "consent" exists. I expect a court would find the matter non-justiciable, as being a political question.
The people have the power to choose Congress.
Although not explicit in the wording of the Constitution, I believe that we can agree that, at a minimum, consent would be withheld unless a majority of senators believed that a candidate was qualified. In other words, being qualified is a necessary condition for approval. Over most of this nation's history, being qualified was also a sufficient condition for approval.
Beginning with Robert Bork's nomination, and, thereafter with Clarence Thomas' and lately with Owens, Pickering and Estrada, the democrats have decided to withhold their approval unless an otherwise qualified candidate's views were acceptable to them on a political basis. This had never been the case in the first two centuries of our country. Senator Schumer has admitted that this is now the case, that the democrats have interpreted the sparse "advice and consent" wording to block the nomination of any judge who is not sufficiently pro-abortion, pro-affirmative action, or pro-judicial activism.