It's been done before, says Jean Edward Smith, writing in an Op-Ed in the New York Times:

But the method most frequently employed to bring the court to heel has been increasing or decreasing its membership.

The size of the Supreme Court is not fixed by the Constitution. It is determined by Congress. The original Judiciary Act of 1789 set the number of justices at six. When the Federalists were defeated in 1800, the lame-duck Congress reduced the size of the court to five — hoping to deprive President Jefferson of an appointment. The incoming Democratic Congress repealed the Federalist measure (leaving the number at six), and then in 1807 increased the size of the court to seven, giving Jefferson an additional appointment.

In 1837, the number was increased to nine, affording the Democrat Andrew Jackson two additional appointments. During the Civil War, to insure an anti-slavery, pro-Union majority on the bench, the court was increased to 10. When a Democrat, Andrew Johnson, became president upon Lincoln’s death, a Republican Congress voted to reduce the size to seven (achieved by attrition) to guarantee Johnson would have no appointments.

WHEN a majority of Supreme Court justices adopt a manifestly ideological agenda, it plunges the court into the vortex of American politics. If the Roberts court has entered voluntarily what Justice Felix Frankfurter once called the “political thicket,” it may require a political solution to set it straight.

If the current five-man majority persists in thumbing its nose at popular values, the election of a Democratic president and Congress could provide a corrective. It requires only a majority vote in both houses to add a justice or two. Chief Justice John Roberts and his conservative colleagues might do well to bear in mind that the roll call of presidents who have used this option includes not just Roosevelt but also Adams, Jefferson, Jackson, Lincoln and Grant.