Read the constitution
Article II, Section 2, Clause 2 of the United States Constitution:
(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, and 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.
Republicans, like you just now, were not objecting to specific nominees, but to the very ability of the president to make appointments! Yes the president is stacking the courts, THAT'S THE RESULT OF ELECTIONS. Want to nominate conservatives to the bench? Win elections!