Douglas was re-elected to the House of Representatives in 1846, but the state legislature elected him to the United States Senate in early 1847.
The United States defeated Mexico in the Mexican–American War and acquired the Mexican Cession in the 1848 Treaty of Guadalupe Hidalgo.
After the war, Douglas attempted to avoid the debate over the Wilmot Proviso by immediately admitting the territory acquired from Mexico as one single, huge state.
His proposal would have allowed the inhabitants of the new state to determine the status of slavery themselves, but Northerners and Southerners alike rejected the plan.

In 1850, Senator Henry Clay introduced a multi-part proposal to admit California as a free state, establish the New Mexico and Utah territories, ban the slave trade in the District of Columbia, and pass a more stringent fugitive slave law.
The proposal, which would form the basis of what would eventually be known as the Compromise of 1850, also required Texas to cede its claims on New Mexico in return for debt relief.
After the apparent collapse of the bill, Clay took a temporary leave from the Senate, and Douglas took the lead in advocating for a compromise based largely on Clay's proposals.
Rather than passing the proposals as one bill, as Clay had originally sought to do, Douglas would seek to pass each proposal one-by-one.
The compromise faced strong opposition from Northerners like William Seward, who favored the Wilmot Proviso and attacked the fugitive slave provision, and Southerners like John C. Calhoun, who opposed the creation of new free states.
With the help of President Millard Fillmore, Douglas put together a bipartisan coalition of Whigs and Democrats that passed the compromise in the Senate.
Along with Fillmore and other supporters of the compromise, Douglas's lobbying helped ensure that the compromise also passed the House of Representatives.
Fillmore signed the compromise bills into law, ending the sectional crisis.

Douglas's role in passing the compromise gave him the stature of a national leader, and he enjoyed the support of the Young America movement, which favored expansionary policies.
Douglas helped pass a bill granting rights-of-way to the Illinois Central Railroad, which would connect Chicago to Mobile, Alabama.
He envisioned a transcontinental country connected by railroads and waterways, with Illinois serving as the gateway to the West. "
There is a power in this nation greater than either the North or the South...that power is the country known as the great West," he stated.
Though he publicly denied interest in running in the 1852 presidential election, Douglas worked behind the scenes to build a base of support.
The 1852 Democratic National Convention held several presidential ballots, with delegates split between Douglas, former Secretary of State James Buchanan of Pennsylvania, 1848 presidential nominee Lewis Cass of Michigan, and former Secretary of War William L. Marcy of New York.
Nomination required the support of two-thirds of the delegates, and none of the major candidates won that level of support.
On the 49th ballot, the convention nominated a dark horse candidate, former Senator Franklin Pierce of New Hampshire.
Despite his disappointment at losing the nomination, Douglas campaigned for Pierce across the Midwest.
Pierce went on to defeat the Whig candidate, Winfield Scott, in the 1852 presidential election, while Douglas won re-election to the Senate.
Douglas was re-elected to the House of Representatives in 1846, but the state legislature elected him to the United States Senate in early 1847. The United States defeated Mexico in the Mexican–American War and acquired the Mexican Cession in the 1848 Treaty of Guadalupe Hidalgo. After the war, Douglas attempted to avoid the debate over the Wilmot Proviso by immediately admitting the territory acquired from Mexico as one single, huge state. His proposal would have allowed the inhabitants of the new state to determine the status of slavery themselves, but Northerners and Southerners alike rejected the plan.
In 1850, Senator Henry Clay introduced a multi-part proposal to admit California as a free state, establish the New Mexico and Utah territories, ban the slave trade in the District of Columbia, and pass a more stringent fugitive slave law. The proposal, which would form the basis of what would eventually be known as the Compromise of 1850, also required Texas to cede its claims on New Mexico in return for debt relief. After the apparent collapse of the bill, Clay took a temporary leave from the Senate, and Douglas took the lead in advocating for a compromise based largely on Clay's proposals. Rather than passing the proposals as one bill, as Clay had originally sought to do, Douglas would seek to pass each proposal one-by-one. The compromise faced strong opposition from Northerners like William Seward, who favored the Wilmot Proviso and attacked the fugitive slave provision, and Southerners like John C. Calhoun, who opposed the creation of new free states. With the help of President Millard Fillmore, Douglas put together a bipartisan coalition of Whigs and Democrats that passed the compromise in the Senate. Along with Fillmore and other supporters of the compromise, Douglas's lobbying helped ensure that the compromise also passed the House of Representatives. Fillmore signed the compromise bills into law, ending the sectional crisis.
Douglas's role in passing the compromise gave him the stature of a national leader, and he enjoyed the support of the Young America movement, which favored expansionary policies. Douglas helped pass a bill granting rights-of-way to the Illinois Central Railroad, which would connect Chicago to Mobile, Alabama. He envisioned a transcontinental country connected by railroads and waterways, with Illinois serving as the gateway to the West. "There is a power in this nation greater than either the North or the South...that power is the country known as the great West," he stated. Though he publicly denied interest in running in the 1852 presidential election, Douglas worked behind the scenes to build a base of support. The 1852 Democratic National Convention held several presidential ballots, with delegates split between Douglas, former Secretary of State James Buchanan of Pennsylvania, 1848 presidential nominee Lewis Cass of Michigan, and former Secretary of War William L. Marcy of New York. Nomination required the support of two-thirds of the delegates, and none of the major candidates won that level of support. On the 49th ballot, the convention nominated a dark horse candidate, former Senator Franklin Pierce of New Hampshire. Despite his disappointment at losing the nomination, Douglas campaigned for Pierce across the Midwest. Pierce went on to defeat the Whig candidate, Winfield Scott, in the 1852 presidential election, while Douglas won re-election to the Senate.
Stephen A. Douglas