In 1846 Lincoln moved from the Illinois State Legislature to the United States Congress, defeating the evangelist Peter Cartwright in Illinois' Seventh District. The victory marked a major step in Lincoln's political career, and introduced him to the national political stage. But the campaign also brought new scrutiny to Lincoln's beliefs.

The contest brought Lincoln's religious ideas to light when Cartwright accused Lincoln of unbelief. Lincoln's detailed rebuttal marked him as a deist, if not a religious skeptic. But the issue also reflected antebellum America's profoundly religious cast. In the 1830s and 40s many Americans returned to Protestant faiths in a broad-based religious uprising that historians have come to call the Second Great Awakening. Mirroring a similar Puritan revival (dubbed the Great Awakening), this movement grew from the efforts of revivalists like Cartwright and Charles Grandison Finney.

Many Protestants believed that they could reform American society, and formed voluntary associations devoted to causes like Sabbath observance and temperance. These groups often provided women with indirect political influence, especially in issues regarding morality, children and the home. The Cartwright-Lincoln contest proved to be an exception to the Second Great Awakening's more general political dynamic, as Whigs largely acted as activist Protestants' spokesmen, while Democrats often resisted such reforms as meddling threatening Americans' liberties.

The Mexican War dominated Lincoln's term in the United States Congress. President James K. Polk renewed the Jacksonian Democrats' drive for additional western lands by turning a simmering border dispute with Mexico into a full-scale conflict. American armies marched to Mexico City and defeated Mexican troops.

Lincoln became one of the Mexican War's leading opponents. His protests failed to bring American armies home, and the American victories produced a huge cession of Mexican lands.

These lands, comprising much of the present-day southwestern United States, began to bring the issue of slavery to a head in American politics. While slave and free states had existed in an unstable political equilibrium for decades, the new territories raised the question: would they be slave or free?

This issue animated the next decade's political debates and precipitated the sectional crisis that eventually led to the Civil War.