Although most northern and southern settlers who entered the new Kansas Territory after May 1854 cared little about slavery and shared a common desire to keep all blacks, free or slave, out of it, turmoil over slavery quickly developed that made a mockery of the Democratic formula of popular sovereignty. Determined to get control of the new territorial legislature for proslavery elements, Missouri residents, quickly labeled "Border Ruffians" and led by former Missouri Senator David R. Atchison, poured across the border to stuff ballot boxes with fraudulent votes for proslavery men when the first legislature was elected. Missourians, not actual residents of Kansas, that is, elected the majority of the first legislature, and that majority quickly passed a series of proslavery laws, including one that required the members of the legislature itself to take an oath vowing that they favored the establishment of slavery in Kansas. Refusing to do so, northern legislators withdrew to Topeka and established a rival, if legally unofficial, "free state" government.

The leading newspaper of the "free state" men was located in Lawrence, and in May 1856 a posse sent by the official proslavery legislature that included Missourians intent on silencing it invaded Lawrence, destroyed its presses, and burned some buildings, but, importantly, killed no one. Nonetheless, eastern Republican newspapers labeled this incident "The Sack of Lawrence" with headlines like "The War Actually Begun — Triumph of the Border Ruffians — Lawrence in Ruins — Several Persons Slaughtered — Freedom Bloodily Subdued." Thus "Bleeding Kansas" became an issue Republicans used during the 1856 presidential campaign, although Republican papers carefully refrained from reporting on a genuine slaughter — the murder of five nonslaveholding Southerners loyal to the proslavery legislature by the antislavery fanatic John Brown along Pottawatomie Creek a few weeks after the "Sack of Lawrence."

Although the antagonism between the "free state" and proslavery governments of Kansas rarely resulted in violence, it remained a profound embarrassment to Democrats if only because it was such a potent propaganda weapon for Republicans. Thus the new Democratic President James Buchanan made readying Kansas for admission to statehood his top priority during 1857. Yet even here the antagonism in Kansas made a shambles of popular sovereignty. Free state settlers boycotted the election for delegates to a convention at Lecompton to write a new state constitution as well as the initial referendum on it. Thus the convention wrote a proslavery constitution which was ratified by some 6000 voters. Yet at another referendum in which free state residents now participated, over 10,000 votes were cast against it. Although a clear majority of Kansas residents rejected the Lecompton Constitution, in December 1857 Buchanan called on Congress to admit Kansas as a slave state under it. This was too much for Stephen A. Douglas, author of the Kansas-Nebraska Act, who broke with Buchanan and urged northern Democrats in the House and Senate to join Republicans against Kansas's entry as a slave state. Ultimately this opposition forced yet another referendum to be held in Kansas on the Lecompton Constitution in August 1858, when it was decisively defeated, thereby permanently burying any possibility that Kansas would become a slave state. Eastern Republicans, in turn, were so impressed by Douglas's stand against Lecompton that they urged Illinois Republicans not to oppose Douglas's reelection to the Senate by the legislature to be elected in 1858. That pressure so angered Illinois Republicans that they took the highly unusal step at their state convention in the summer of 1858, prior to the fall legislative elections, of announcing that Abraham Lincoln was the Republican candidate for U.S. Senator. This commitment set the stage for the famous Lincoln-Douglas debates that year.