Abraham Lincoln's political career began after he attained a law license and was elected to the Illinois General Assembly, where he served four terms before a single term in the United States House of Representatives. After returning to private law practice and becoming a prosperous and skilled attorney, he was drawn back into politics by the passage of the Kansas-Nebraska Act. Lincoln first challenged incumbent James Shields in 1856 for one of Illinois' seats in the United States Senate, but when a deadlock in the Illinois General Assembly made it likely that a pro-Douglas man would win, he threw his support to fellow anti-Nebraska candidate Lyman Trumbull. That willingness to set aside his own ambition in support of opposition to the further spread of slavery helped make him the leading contender to face Stephen A. Douglas in 1858 when Douglas sought re-election. Nominated by the newly formed Republican Party in Illinois, Lincoln became an inviting target for critics, in part because he had opposed the Mexican-American War while serving in the United States House of Representatives, and hostility toward him was evident in some Democratic circles as divisions also appeared within the Illinois Democratic Party. In that 1858 campaign, Lincoln argued against slavery with an eloquence that had not characterized his earlier political career.