Two clauses in the Constitution describe how, and for how long, members of the judiciary are to be appointed. The “appointments clause” in Article II, Section 2, Clause 2 empowers the president to nominate and, with the advice and consent of the Senate, appoint public officials. Article III, Section 1 states that “judges, both of the supreme and inferior courts, shall hold their offices during good behaviour.”

We believe that our proposal to limit the terms of future Supreme Court justices to a single 18-year term is consistent with those clauses, and below we describe how.

“Good behaviour” clause:
Under our proposal, future Supreme Court justices would retain their “office” as “justices” for life, albeit as “senior justices,” with the same compensation and with the ability to return to high court service if so designated. We know that Congress has the authority to designate this kind of semi-retired status to an Article III officer since it’s already done so, having created the position of “senior judge” in 1919, which is universally regarded as constitutional.

Further, in 1937, Congress permitted retired members of the Supreme Court to sit by designation on lower courts, and 11 retired justices have availed themselves of this post-retirement service option. Retired justices sitting on lower courts remain “justices” by definition in terms of title and compensation, so even post-retirement, they are “hold[ing]” their “office.”

For example, whenever David Souter hears cases on the First Circuit, or Sandra Day O’Connor heard cases on the Ninth Circuit, they were “Justice Souter” and “Justice O’Connor” during arguments and on written opinions. Further, Justice John Paul Stevens believed that a return to high court service, i.e., a form of semi-retirement, could be implemented by statute.

Appointments clause:
There’s not as much scholarship on how the appointments clause would interact with this proposal, but that’s mainly since there’s almost no direct impact: a president under our proposal would retain his or her Article II ability to appoint justices, albeit in every first and third year of his term, and the Senate would still provide its advice and consent – so there’s no conflict.
There are other proposals to end life tenure at the court that would compel a Senate vote on a nominee or create a statutory judicial nominating commission, but we feel that such proposals do not pass constitutional muster.

Here are scholars’ takes on constitutionality:
Yale Law’s Judith Resnik (Jan. 2005; link): “To alter the import of life tenure, Congress could create incentives for judges to shorten their length of service and the Court could reread the meaning of ‘good behavior’ to sanction a term limit” (p. 5).

“Just as over this past century reinterpretation has permitted much of the ‘judicial Power of the United States’ that the words of the Constitution appear to mandate vesting in courts with life-tenured judges to be delegated to non-life-tenured jurists in courts and in agencies, Article III could similarly be reinterpreted to require guaranteed terms yet also to permit a mandatory, statutorily-fixed retirement age [or term limit]. Congress could enact such a statute with prospective application, such that current judges would not lose their seats, thereby avoiding any arguments that it would diminish the salary or otherwise impair the independence of sitting jurists” (p. 36).

National Review’s John Fund (11/24/19, link): “The [18-year term limits] proposal could be enacted without amending the Constitution. Article III, Section 1 states that ‘Judges, both of the supreme and inferior Courts, shall hold their Offices during good Behavior.’ This has been interpreted to mean that Supreme Court justices have a life tenure. But the Constitution is silent on what is meant by ‘Offices.’ Nothing is said about judges remaining at their original posts for life.”

Penn Law’s Kermit Roosevelt and Ruth Helen-Vassilas (9/30/19, link): “Federal judges are constitutionally entitled to ‘hold their offices’ during good behavior and not have their salaries reduced. [The 18-year term limits] plan does not diminish salaries, and it is consistent with a current U.S. law (28 US Code § 371(b)) that states explicitly that district and circuit judges who take senior status ‘retain the office.’ It follows that our legislators can assign senior status to justices, as well.”

Yale Law’s Bruce Ackerman (12/20/18, link): “While the Constitution guarantees justices tenure for life, it nowhere states that they must serve their entire term on the Supreme Court. Nothing prevents Congress from limiting the justices’ service to [any number of] years and then providing them with a position on an appellate court for the rest of their lives.”