The material elements of the problem, then, were comparatively simple, and the immediately pressing questions were easily phrased; but the intangible element of public opinion was uncommonly hard to estimate. So far as the great parties were concerned, it was impossible to fix upon either of them any general theory about slavery or any definite policy with it. Up to this time, both had apparently gone on the understanding that it was not a proper issue in political contests. A small group of unpractical men had, in fact, tried to build up a party on the issue of opposition to it, but they had no prospect of carrying a single electoral vote. The adherents of the old parties were agreed on one thing: that there was no lawful way for Congress or the people of the free States to interfere with slavery in the slave States. They were divided among themselves, inside of party lines, on the fugitive slave law, on the interstate slave trade, on slavery and the slave trade in the District of Columbia, and on slavery in the Territories.

But if party lines did not yet accurately represent the divisions of opinion on these questions, there was, nevertheless, a grouping of men according to their opinions on the general question which already had its effects in politics. Every thoughtful American of that day belonged to one or another of several groups according to the view he took of two things: slavery itself, and the body of law and usage that had grown up about it. There were the abolitionists, who believed slavery to be so utterly wrong that they were ready to go all lengths to get rid of it, violating the Constitution, breaking the compromises, endangering the Union. There were the Southern fire-eaters, who not only believed slavery right but were similarly willing to go all lengths to defend and extend it. There were the moderate men who made up the bulk of the two great parties in the North, who believed slavery wrong but felt themselves bound by the compromises of the Constitution which protected it where it already existed and debarred from any method of attacking it which might bring the Union into danger. There were the moderate men of the South, Whigs and Democrats alike, who believed either that slavery was right or at least that there was no better state possible for the mass of the blacks, but who were yet devoted to the Union and respected their constitutional obligations. Finally, there were men so constituted that they could decline to take any thought whether slavery were right or wrong, and could deal with every question that arose concerning it as a question of expediency merely, or of law and precedent.

To which of these groups should Douglas join himself? Up to this time, his public record was too meagre to show clearly where he stood. In 1845, when the bill to annex Texas was before the House, he had offered an amendment extending the compromise line of 1820 through the new State, so that if Texas were ever divided slavery would be prohibited in such State or States as should be formed north of that line. Both in the House and in the Senate he had voted against the famous resolution of Mr. David Wilmot to exclude slavery from any territory that we might get from Mexico, and he continued to oppose that motion, in whatever form it appeared, until the legislature of Illinois instructed him to favor it. In 1848, he voted for the so-called Clayton Compromise, which proposed to organize California, Oregon, and New Mexico into Territories and merely extend over them the Constitution and laws of the United States so far as these should prove applicable; but he also voted for the bill to organize the Territory of Oregon with a clause prohibiting slavery. By his speeches, no less than by his votes, he was committed to the position that the Missouri Compromise was a final settlement so far as the Louisiana Purchase was concerned, and that the compromise line ought to be extended through the Mexican Cession to the Pacific. He was not clearly committed on any other of the points at issue between the friends and the opponents of slavery.

But he had roundly denounced the abolitionists, and he had married the daughter of a slaveholder. The day after his wedding his father-in-law presented him a deed to a plantation in Mississippi and a number of slaves. He gave it back, not, so he declared, because he thought it wrong to hold slaves, but because he did not know how to govern them or to manage a plantation. His wife soon fell heir to the land and negroes, and at her death they passed to her children under a will which requested that the blacks be not sold but kept and cared for by the testator's descendants. Douglas, as the guardian of his infant children, respected their grandfather's wishes. For that reason he was called a slaveholder, and a fellow senator once openly accused him of shaping his course as a public man to accord with his private interests. He denied and disproved the charge, but proudly added: "I implore my enemies, who so ruthlessly invade the private sanctuary, to do me the favor to believe that I have no wish, no aspiration, to be considered purer or better than she who was, or they who are, slaveholders."

He was of those who could be indifferent to the moral quality of slavery. He could favor whatever policy the Constitution required, or precedents favored, or public expediency demanded; if his enemies were to be believed, he could take whatever course ambition and self-interest impelled him to. Never once during his long wrestling with the slavery question did he concede that any account should be taken of the moral character of the institution, or intimate that he believed it wrong for one man to hold another man in bondage.

The Democratic National Convention of 1848, though its platform was as vague as it could be made, nominated a candidate who was committed to a particular plan with slavery in the Territories. The candidate was Lewis Cass, of Michigan, and his plan was set forth in a letter to one Nicholson, of Nashville, Tennessee, of date December 24, 1847. The plan appeared to be a very simple one. It was to leave the people of each Territory, so soon as it should be organized, free to regulate their domestic institutions as they chose. He favored it for two reasons: first, because Congress had no right to interfere; and second, because the people themselves were the best judges of what institutions they ought to have. That was the barest form of the doctrine which its opponents in derision named "squatter sovereignty." It was contrary to the doctrine of the Wilmot Proviso, which invoked the authority of Congress to exclude slavery from all the Territories, and contrary, also, to whatever doctrine or no doctrine was implied in the motion to extend the compromise line to the Pacific, exercising the authority of Congress to exclude slavery north of the line and forbearing to exercise it south of the line. It was equally contrary to a third doctrine which was brought before the convention. William L. Yancey, a delegate from Alabama, offered a resolution to the effect that neither Congress nor any territorial legislature had any right to exclude slave property from the Territories. This was a mild statement of the extreme Southern doctrine that slaves were property, so recognized by the Constitution, and that a slaveholder had the right to take his slaves anywhere but into a State where slavery was forbidden.

The doctrine of Cass seemed to accord best with that democratic theory of the government which Douglas had always professed. It accorded well with his faith in the builders of the West. It alone, of all the doctrines advanced, accorded fully with his attitude of indifference to the moral quality of slavery. He soon embraced it, therefore, and for the rest of his life he was oftenest occupied embodying it in legislation, defending it, restating it to suit new conditions, modifying it to meet fresh exigencies. Cass, though his authorship of the doctrine is disputed, was at first held responsible for it, and he advocated it with great ability. But in the end men well-nigh forgot who the author of the principle was, so preëminent was Douglas as its defender. He made it his, whosesoever it was at first, and his it will always be in history.

During the session of 1848–49, he introduced a bill to admit California as a State, leaving the people to settle the slavery question as they pleased. But his first great opportunity came in the session of 1849–50.

Cass had been beaten in the election. Zachary Taylor, the successful candidate of the Whigs, was a Southerner and a slaveholder, but he was elected on a non-committal platform, and he had never declared, if indeed he had ever formed, any opinions on the questions in dispute. His first message merely notified Congress that California, whither people were rushing from all parts of the country in search of gold, had of her own motion made ready for statehood; he expressed a hope that New Mexico would shortly follow her example, and recommended that both be admitted into the Union with such constitutions as they might present. Immediately, the House, where the free-soilers held a balance of power, fell into a long wrangle over the speakership; and the Senate was soon in fierce debate over certain anti-slavery resolutions presented from the legislature of Vermont. The North seemed to be united on the Wilmot Proviso as it had never before been united on any measure of opposition to slavery, and the South, fearing to lose the fruits of her many victories in statesmanship, in diplomacy, and on Mexican battlefields, was threatening disunion if, by the admission of California as a free State with no slave State to balance, her equality of representation in the Senate should be destroyed. The portents were all of disagreement, struggle, disaster.