After Congress passed an enabling act, Illinois became the 21st state admitted to the Union. The state's first two senators, Jesse Burgess Thomas and Ninian Edwards, both of Edwardsville, took their seats and were sworn into office on December 4. They drew lots to determine their class assignments. Edwards drew Class 1, with a term to expire March 3, 1819. Thomas drew Class 3, with a term to expire March 3, 1823.
Congress agreed to the Missouri Compromise, which admitted Maine as a free state and allowed the Territory of Missouri to draft a constitution and form a state government without restrictions on slavery. Illinois senator Jesse Thomas introduced the provision which prohibited slavery north of Missouri's southern boundary, with the exception of Missouri.
Former senator Ninian Edwards took office as the state of Illinois's third governor. He held the position for four years.
Kaskaskia lawyer and future senator James Shields of Belleville nearly engaged in a duel with Abraham Lincoln. Shields had challenged Lincoln after Lincoln penned a letter published in a newspaper that poked fun at Shields, who then served as the state auditor. As Illinois state law prohibited dueling, the two men met across the border to resolve their dispute. After friends intervened, both men resolved their disagreement and a friendship developed.
Having been denied a seat in the Senate months earlier on grounds that he had not been a U.S. citizen for the required nine years, James Shields again won election to the Senate. This time, the Senate accepted his credentials, seating him on December 3. Losing a bid for reelection in 1855, he subsequently won a Senate seat in Minnesota and later in Missouri, becoming the only person to represent three states in the Senate.
Senator Henry Clay of Kentucky introduced in the Senate a series of resolutions that sought to resolve the question of slavery in the southwestern territories acquired after the Mexican War. These resolutions became known as the Compromise of 1850. After Congress rejected Clay's omnibus bill, Senator Stephen A. Douglas of Chicago, negotiating quietly, helped ensure congressional passage of the compromise resolutions individually.
After months of bitter debate, the Senate passed Senator Stephen A. Douglas's bill organizing the territories of Kansas and Nebraska. The Kansas-Nebraska Act was highly controversial because it replaced the Missouri Compromise's geographical restriction on slavery with "popular sovereignty," allowing settlers of the territory to decide whether to permit or prohibit slavery.
The Senate voted to uphold the claim of Lyman Trumbull of Alton to his Senate seat. He had been elected by the state legislature on February 8, 1855, but opposition Democrats claimed that he was ineligible for the Senate seat. The challenge to Trumbull's credentials provoked heated debates over the nature of states' rights.
The Illinois state Republican convention nominated Abraham Lincoln to challenge Democratic senator Stephen A. Douglas for his seat in the U.S. Senate. This was the first time a political party, meeting in state convention, had endorsed a candidate for the U.S. Senate, thereby infringing on the state legislature's constitutional responsibilities. Receiving his party's nomination, Lincoln then delivered his immortal "House Divided" speech.
Campaigning for a U.S. Senate seat from August to October, Abraham Lincoln and Senator Stephen A. Douglas debated each other in seven Illinois towns. In the so-called Lincoln-Douglas debates, the two candidates discussed their views on slavery: Lincoln favored abolition of slavery, while Douglas spoke in support of popular sovereignty, believing settlers should decide for themselves whether to prohibit or allow slavery to exist. Several months after the debates, the state legislature met to exercise its responsibility to fill the Senate seat and reelected Douglas by a margin of 54 to 46. About that experience, Lincoln commented to a friend, "It hurts too much to laugh and I am too big to cry."
Pro-slavery Democratic senators stripped Senator Stephen A. Douglas of his chairmanship of the Committee on Territories in retaliation for his "moderate" views on slavery.
Senator Stephen A. Douglas died of typhoid fever in Chicago.
Senator John Alexander Logan of Chicago became chairman of the Committee on Military Affairs (today's Committee on Armed Services), serving until 1877. He chaired the committee again from 1881 to 1886.
After serving only 10 days, Governor Richard Oglesby (who had previously served as governor from 1865 to 1869) resigned his seat having been elected to the U.S. Senate. Oglesby later served a third time as governor from 1885 to 1889.
Supreme Court justice David Davis of Bloomington resigned from the court after winning election as an independent to the U.S. Senate. He did not seek reelection in 1882 and left the Senate in March 1883.
David Davis was elected Senate president pro tempore. With the office of vice president vacant, President pro tempore Davis stood next in line of succession to the presidency. He held that post until he left the Senate in March 1883.
Senator John A. Logan was nominated for vice president of the United States at the Republican Party Convention held at Exposition Hall in Chicago. His presidential running mate was James G. Blaine of Maine. They lost to Democratic presidential candidate Grover Cleveland and vice presidential candidate Thomas A. Hendricks, a former senator from Indiana.
The Senate declared the election of William Lorimer of Chicago invalid over allegations that he had obtained his seat by bribery and corruption. The resulting storm of public outrage over the Lorimer case, combined with an infusion of recently elected progressive minded members, led the Senate on June 12, 1911, to approve a long pending constitutional amendment providing for direct popular election of senators.
Upon his retirement, Senator Shelby Cullom established a 30-year service record, making him the longest-serving senator in Illinois history.
The Seventeenth Amendment to the U.S. Constitution went into effect allowing the direct election of U.S. senators. Lawrence Yates Sherman of Springfield, who had been elected to the Senate by the state legislature in 1913, was Illinois's first popularly elected senator, winning election in 1914.
The Senate rejected the credentials of Frank L. Smith of Dwight following charges that he accepted campaign donations from the Illinois public utilities he oversaw as chairman of the Illinois Commerce Commission. Subsequently, he tendered his resignation.
With the sudden death of Senator J. Hamilton Lewis, the Senate recessed for the day. In honor of the Illinois senator, his funeral service was held in the Senate Chamber on April 12, with President Franklin Roosevelt, justices of the Supreme Court, and other dignitaries in attendance.
Without ceremony, a portrait of the late J. Hamilton Lewis by Louis Betts was hung in the U.S. Capitol. The portrait was presented by the senator’s widow to the Joint Committee on the Library shortly before it was exhibited in the Senate wing of the Capitol.
The Senate elected Edward F. McGinnis of Illinois sergeant at arms of the Senate. He held that post until January 2, 1949.
Illinois voters elected the first African American woman to the U.S. Senate. Senator Carol Moseley Braun of Chicago, a former prosecuting attorney, had previously served in the Illinois house of representatives from 1978 to 1988.
The Senate elected Terrance W. Gainer of Chicago as its 38th sergeant at arms, a position he held until May 4, 2014. He had previously served as chief of the U.S. Capitol Police.
Senator Barack Obama, who had been nominated as the Democratic presidential candidate in August 2008, became the nation’s first African American to be elected president of the United States. Obama’s running mate was Senator Joseph R. Biden Jr. of Delaware. Obama and Biden were sworn into office on January 20, 2009.