Lincoln had then reached the full maturity of his powers. His equipment as a statesman did not embrace a comprehensive knowledge of public affairs. What he had studied he had indeed made his own, with the eager craving and that zealous tenacity characteristic of superior minds learning under difficulties. But his narrow opportunities and the unsteady life he had led during his younger years had not permitted the accumulation of large stores in his mind. It is true, in political campaigns he had occasionally spoken on the ostensible issues between the Whigs and the Democrats, the tariff, internal improvements, banks, and so on, but only in a perfunctory manner. Had he ever given much serious thought and study to these subjects, it is safe to assume that a mind so prolific of original conceits as his would certainly have produced some utterance upon them worth remembering. His soul had evidently never been deeply stirred by such topics. But when his moral nature was aroused, his brain developed an untiring activity until it had mastered all the knowledge within reach. As soon as the repeal of the Missouri Compromise had thrust the slavery question into politics as the paramount issue, Lincoln plunged into an arduous study of all its legal, historical, and moral aspects, and then his mind became a complete arsenal of argument. His rich natural gifts, trained by long and varied practice, had made him an orator of rare persuasiveness. Abraham Lincoln: An Essay Emphasis added
Numerically most of Lincoln's public utterances after 1854 relate to the question of slavery expansion. I'd guess that the same was true of his private papers. And these words were more emotionally and intellectually alive than his routine, "perfunctory" remarks on tariffs. It's hard for a surface examination over a century later to pick up just what's the standard political "boiler plate" rhetoric of the day and what reflects real passion and emotional involvement, but Schurz and others were there at the time and their judgement should be taken into account.
I suppose the response is that talk about slavery was all window dressing for an economic agenda. If you believe that the Civil War absolutely has to have been all about tariffs, you'll dismiss everything else as mere rhetoric. The idea seems to be that there has to be a dirty truth behind "high-flown rhetoric." But 19th century America was a highly rhetorical. Political and moral abstractions were taken seriously. And why would those who weren't prejudiced in that direction find the tariff behind slavery? Why not slavery behind the tariff? It certainly seems hypocritical and unfairly selective to attack attempts to look beneath the veneer of Confederate rhetoric while reducing the Union cause to dollars and sense.
At the time, the Republicans were accused of deception and having a secret agenda, but the charge wasn't so much that they were using the issue of slavery extention to slip in a protectionist agenda. It was rather the reverse: that they used the tariff issue to win over Pennsylvanians and other key voters to an anti-slavery agenda. Southerners hated tariffs, but they were apart of normal political debate. Opposition to slavery was seen as a threat to their way of life. That's what Thomas Clingman thought.
From today's point of view the Republican refusal to attack slavery where it already existed was a compromise with evil, but for many Southerners at the time, opposition to slavery in the territories and to the Fugitive Slave Act was the beginning of the end for slavery. Today, almost all people accept that slavery was wrong and should be illegal. A century and a half ago, that was a very controversial position, far more controversial than protection vs. free trade. It's only because the slavery question has been resolved that some have forgotten how violent the conflict was.
Debate on the tariff was loudest on a few types of goods. In the 1850s it was wool and iron. There was a natural opposition between those who produced wool and pig iron (pro-tariff) on the one hand and those who spun woolen goods or forged iron products (anti-tariff). It was only because slavery was so much more important an issue to people that the natural free trade coalition between Western and Southern farmers and their Eastern allies broke down.
To Clingman and many other secessionists, protective tariffs were tariffs were theft, but so was the refusal to return runaway slaves, and the threat of abolition, however remote, was highway robbery and the destruction of their society. If I'd been alive then and in their position, I might have felt the same, but that doesn't mean that it was right or justified, and it doesn't excuse lying and denial now.
Schurz - "It is true, in political campaigns he had occasionally spoken on the ostensible issues between the Whigs and the Democrats, the tariff, internal improvements, banks, and so on, but only in a perfunctory manner. Had he ever given much serious thought and study to these subjects, it is safe to assume that a mind so prolific of original conceits as his would certainly have produced some utterance upon them worth remembering. His soul had evidently never been deeply stirred by such topics."
Contrasted with Lincoln: "I was an old Henry Clay tariff whig. In old times I made more speeches on that subject, than on any other. I have not since changed my views" - Lincoln to Edward Wallace, October 11, 1859
In other words, what Schurz calls perfunctory and minimal was said by Lincoln himself to have been his #1 issue in old times.
Now, consider what Schurz says next - that Lincoln had never given tariffs any serious thought. Aside from the fact that Lincoln did give them serious thought to the point that he made more speeches on them than any other issue in his early years (and that he was still making speeches on them in his presidential years) there exists direct documentary evidence that he did give serious thought to tariffs. This is found in the collected works volume I, pages 408-416. These pages show a large fragment of Lincoln's personal notes on protectionism. In them he lays out several arguments in favor of protectionism, indicating that he has put much thought into the issue and his position on it.
Since it is a safe conclusion that Lincoln knew Lincoln better than Schurz knew Lincoln, places where Lincoln contradicts Schurz on the subject of Lincoln must be yielded to Lincoln. Therefore Schurz was wrong. But when his moral nature was aroused, his brain developed an untiring activity until it had mastered all the knowledge within reach . As soon as the repeal of the Missouri Compromise had thrust the slavery question into politics as the paramount issue, Lincoln plunged into an arduous study of all its legal, historical, and moral aspects, and then his mind became a complete arsenal of argument. His rich natural gifts, trained by long and varied practice, had made him an orator of rare persuasiveness.