Actually, he saved it at least three times, I would say. But the one I want to talk about here is his successful face-off with John C. Calhoun and the South Carolina secessionists.
I've written about this subject here before in Andrew Jackson, States Rights and the South 10/24/03. Here I want to draw on Sean Wilentz' Andrew Jackson (2005), one of the American Presidents series edited by Arthur Schlesinger, Jr.
The fight over the Tariff of 1828 became a testing ground for Calhoun and similar advocates of secessionist treason in defense of slavery. As James MacGregor Burns wrote in The Vineyard of Liberty (1981):
For rice and cotton growers, the 1820s were a time of rapid economic change, price and demand instability, credit squeezes, and depression, all tending toward a rising sense of social and economic insecurity, which in turn fostered a powerful parochialism and sectionalism. The Tariff of 1828 excited the worst southern fears; it was to them literally a tariff of abominations, to be despised and shunned. In a decade of peace they could no longer accept the tariff as a defense measure. Federal policy on internal improvements and other questions also continued to antagonize South Carolinians. But behind all the old issues always loomed the specter of northern interference with slavery. An alleged slave conspiracy, led by Denmark Vesey of Charleston, along with rumors of other planned slave revolts, aroused dread over threats from inside; the stepped-up efforts of the American Colonization Society [an anti-slavery group] in the North aroused fears over threats from outside. (my emphasis)
The South Carolina legislature passed the South Carolina Exposition and Protest in 1929, a document written by John C. Calhoun that defended the doctrine of nullifcation, the right of a state to declare a federal law void within that state. Calhoun at that time was Jackson's Vice President and tried to keep his authorship of the document secret.
The confrontation between South Carolina and the federal government under Jackson brought the contradiction between a national Union founded on democratic principles and the reality of slavery into sharper contrast, as would the subsequent confrontations. Jackson was a slaveowner himself and, unlike slaveowner Thomas Jefferson, supported the institution of slavery. But, as Wilentz explains, Jackson was also a staunch Unionist, a democratic nationalist:
Political leaders, including Jackson, strongly suspected Calhoun's authorship of the Exposition, which they interpreted as an effort to consolidate southern support for a later run for the presidency. But Jackson also took the idea of nullification seriously - and as a piece of rank heresy. According to his strict reading of the Constitution, Jackson held that Congress had full and direct authority over the enacting tariffs, including dictating tariff rates. To deny the rights of the majority in Congress to govern as it saw fit was, in this instance, an absurd breach of the Framers' explicit intentions. Worse, talk of nullification, let alone secession, endangered the Union. In Jackson's mind, it was an outrageous affront to the glorious embodiment of the American Revolution. "There is nothing I shudder at more than the idea of the separation of the Union," he had written to a South Carolina leader before the 1828 election. "It is the durability of the confederation upon which the general government is built, that must prolong our liberty. ...[T]he moment it separates, it is gone." (my emphasis)
The confrontation unfolded over years. In 1832, Jackson supported a revision of the "tariff of abominations", without yielding on the critical principle of federal authority. That was the year Jackson won re-election for a second term as President, with Martin Van Buren as his Vice Presidential running mate this time. Calhoun returned to the Senate the following year.
There was also a legislative election in South Carolina. As Wilentz writes, Southern fears over slave revolts and doubts about the survival of slavery had been mounting even before Jackson's Presidency began. He writes:
Events during the early years of Jackson's presidency further convinced slaveholders that their property and their way of life were besieged. Anxiety mounted in 1829 and 1830, when officials in Charleston and other southern seaports intercepted copies of an incendiary pamphlet - written by a Boston-based free black, David Walker, and smuggled south - bidding the slaves to overthrow their masters. A few months later Walker suddenly died, in what looked to some like suspicious circumstances. A short time after that, a white Bostonian, William Lloyd Garrison, established a new radical newspaper, The Liberator, dedicated to bringing about slavery's immediate demise. Just as ominously, antislavery advocates in the Virginia legislature forced a debate over a gradual emancipation plan early in 1832. Although the proposal failed, that the Virginians even discussed abandoning slavery shocked slaveholders in the Deep South, and especially in South Carolina. [Virginia, one disgusted South Carolinian remarked, had become "infested" with "Yankee influence.") Threatened from without and within, slavery's defenders began to see any effort by the federal government to enact policies they deemed unfavorable to the South as part of a larger antislavery design. This included the protective tariff, which one state rights' party convention in South Carolina declared was intended to hasten "the abolition of slavery throughout the southern states." (my emphasis)
It's worth noting that the 1932 Virginia debate over abolishing slavery was the last time such a proposal was so widely discussed in the South. After that time, antislavery talk was increasingly suppressed in the slave states, a definite abridgment of the rights of free speech and the press for white Southerners. This also made the contradiction between a democratic national republic and slavery in the Southern states increasingly unavoidable.
The newly-elected South Carolina legislature pushed the confrontation further by calling a special convention, which in November, 1832 adopted an Ordinance of Nullification. The Ordinance declared the tariffs of 1828 and 1832 null and void in South Carolina and declared that the federal courts had no authority to intervene. The Ordinance also explicitly threatened secession if the Jackson adminitration tried to collect the tariff in South Carolina.
Jackson answered the challenge in December with a "Proclamation to the People of South Carolina" asserting federal authority. He followed it up by getting Congress toadopt the Force Bill in March, explicitly giving him the authority to use force against South Carolina if they attempt to block federal collection of taxes.
Seeing that Old Hickory was dead serious about strangling the secessionist baby in its cradle (as Churchill was later to suggest doing to Soviet Bolshevism), the South Carolina convention backed down and rescinded the Ordinance of Nullification.
Even though the immediate object of the struggle was the "tariff of abominations", it was clear then as it should be now that in this confrontation, the Union and the democratic republic won, slavery and secession lost. The outcome of most of the other slave state/free state confrontations - the Missouri question, the Mexican War, the Compromise of 1850, the Kansas-Nebraska Act, the Dred Scott decision - represented surrenders to the Slave Power. Andrew Jackson's stomping the secession cockroach in South Carolina was a victory for democracy and the Union.