In the case of both Adams and Roosevelt, the system broadly worked to check political power grabs. Congress rectified the court’s size in 1802 and rejected its expansion in 1937.
Today, the situation is quite different. First, the call for a change to the court’s size is not a response to specific rulings that Democrats disagree with. There were few widespread calls for an expanded court following the decisions in District of Columbia v. Heller, which vastly expanded gun-ownership rights, Shelby Co. v. Holder, which gutted the Voting Rights Act, or even Citizens United v. FEC, a ruling so universally reviled by voters that a 2010 Washington Post-ABC News poll found even 76% of Republicans disagreed with it (85% of Democrats and 81% of independents did, too — though many Republican officeholders welcomed the influx of money into campaigns).
What’s really driving the renewed interest in court expansion is something else: the politicized change in the size of the court has already happened. It occurred in 2016, when a Republican-controlled Senate allowed the court to shrink to eight justices. Not only did the Senate fail to fulfill its constitutional duty to vote on the president’s nominee, some Senate Republicans were prepared to keep the court at eight if Hillary Clinton won the 2016 election. Sen. Ted Cruz and the late Sen. John McCain both floated that possibility in October 2016, with Cruz musing, “There is certainly long historical precedent for a Supreme Court with fewer justices.”