A U.S. President that did not like the pro-slavery U.S. Constitution could actively seek to overthrow the U.S. Constitution. He could encourage many John Brown-type murder raids in the South.
He could encourage northern states to provide sanctuaries to financiers of the murder raids.
He could encourage Congress to pass confiscatory import taxes aimed at destroying the economies of the southern states.
He could use U.S. Navy warships to create a pretext for war such as the Gulf of Tonkin incident, errrr . . . I mean the Fort Sumter incident.
Somewhat like presidents who didn’t like freedom used their power to strengthen the pro-slavery side? Like what Democrat presidents had been doing for decades before the Civil War?