John Waugh argues that the five years Lincoln spent as a circuit judge for the Illinois Eighth Judicial Circuit were the most important for Lincoln's political career even though he was inactive:
As he practiced law in Springfield and rode the circuit, the lethal issue of slavery continued to tear at the life of the Union. “The world is dead to hope,” Lincoln told Herndon, “deaf to it own death struggle made known by a universal cry. What is to be done? Who can do anything and how can it be done? Did you ever think on these things?”
For the next five years he, for one, thought deeply on those things—had them in soak. And as the great issue was thundering toward its crescendo he emerged fully armed and ready, with as firm a grip on the issue as anybody in the country. From this soaking the Lincoln who would save the Union emerged in the American consciousness. Unknown perhaps even to him, he was in those five critical years preparing himself for greatness.
He read intently and thought deeply, reading newspapers voraciously, driving Herndon nearly out of his mind by reading them aloud in the law office, so as to “catch the idea by two senses”—simultaneously hearing and seeing it. That way Lincoln told Herndon, “I remember it better, if I do not understand it better.”
During this time, his political rival, Douglas, was deep into the slavery issue:
As Lincoln had all this in soak, his long time rival in Illinois, Senator Stephen A. Douglas, was taking the slavery issue violently to a new level. Douglas had been the chief architect or the Compromise of 1850, which had clamped a temporary lid on the simmering issue of slavery in the territories. Douglas believed that the answer, the cure-all, to slavery agitation in America was “popular sovereignty”—letting every territory decide for itself whether it would enter the Union a free state or a slave state. Douglas claimed not to care if slavery was voted up or down, as long as the decision accurately mirrored the will of the people of the territory.
His conclusion is that Lincoln's time to read and think allowed him to emerge ready to be the politican who could win in 1860:
Lincoln dogged Douglas as he stumped for support of his new act in Illinois in 1854, speaking after Douglas spoke, refuting his pro-Kansas-Nebraska arguments on every possible platform. Newly joined to the newly formed Republican Party, Lincoln became the obvious choice to run against Douglas for his Senate seat in 1858. Together those two giants from Illinois waged the great debates over the slavery issue that immortalized them in American political history. Lincoln lost to Douglas in that campaign. But two years later, now an acknowledged political power in his own right, a man who understood the issue and could stand toe-to-toe to Douglas, it would he, not Douglas who would be elected president.
Those five years then, in which he had thought himself dead in politics and had his career in soak, are what in the end ultimately carried Lincoln to the very pinnacle of political power.