An old post of mine at
https://www.alternatehistory.com/forum/threads/seward-in-1860.397598/#post-13075387 (my apologies for any links that may no longer work):
***
Let's say Lincoln is unavailable for some reason or other in 1860--dies in a railroad accident or whatever.
Seward is probably the most likely alternative as Republican presidential nominee, despite his disadvantages (especially the distrust of him by the ex-Know Nothing element in the Republican Party) though I couldn't rule out the Republican national convention selecting another candidate, perhaps Lincoln's fellow-Illinoisan Trumbull.
But let's say Seward does get nominated. Three questions:
(1) Will he win? Almost certainly, yes. It was the election of 1858, not that of 1860, which showed that the country was ready to elect a Republican president. In particular, the Democratic Party was shattered in Buchanan's own state of Pennsylvania, one of the keys to Buchanan's victory in 1856. The state's delegation to the US House went from 15 Democrats and 10 Republicans after the 1856 election to 20 Republicans and 5 Democrats after the 1858 one--and two of those five were anti-Lecompton Democrats.
By 1860 any plausible Republican presidential candidate would probably have carried Pennsylvania. The only states that Lincoln won really narrowly that year were California (4 electoral votes), Oregon (3), Illinois (11), and the four electoral votes he got in New Jersey. If we assume that Seward would lose all 22 of these electoral votes, he would still have 158, six more than were needed to win.
http://en.wikipedia.org/wiki/United_States_presidential_election,_1860
For Seward to fall short of an Electoral College majority, he would have to lose either New York, which Lincoln carried (against a "fusion" ticket) by 7.4 points or Indiana which Lincoln carried by 8.7. Neither of these seems likely to me. Apart from Seward's home-state advantage, New York had already gone for Fremont in 1856, and the Republicans also carried the state in 1858 despite the failure to arrange a joint ticket with the Americans and despite the presence of a radical abolitionist ticket headed by Gerrit Smith.
https://en.wikipedia.org/wiki/New_York_state_election,_1858 As for Indiana, it would be close in a two-man race, but Senator Jesse Bright was bitterly opposed to Douglas, and as a result Breckinridge got 4.5 percent of the vote there, more than in any other state of the Old Northwest. This would be more than enough to assure the election of any Republican presidential candidate--and Seward after all would not be *that* much weaker a candidate than Lincoln. (Some of the former Know Nothing element of the Republicans did dislike Seward, but to compensate he might get more immigrant votes.)
(2) Could President-Elect Seward have prevented secession? Short answer: Not of South Carolina and probably not of the rest of the Deep South. Southerners still remembered his "higher law" and "irrepressible conflict" speeches. They found his attempt to moderate his image in 1860 unconvincing. Indeed, much of the secessionist rhetoric of 1860 was an attempt to convince Southerners that Lincoln was *just as bad* as Seward.
It is true that Seward's *tone* following Lincoln's election in OTL was conciliatory. But in substance he was not making any concessions on the slavery issue. As James Oakes writes in *Freedom National: The Destruction of Slavery in the United States, 1861-1865*, pp. 65-66
https://books.google.com/books?id=MaVp-YES1F0C&pg=PA65
"...Charles Francis Adams, who along with Seward was a leading Republican supporter of conciliation, freely admitted that his moderate stance toward the South did not mean he was willing to compromise basic Republican principles. If anything, Adams and Seward were willing to conciliate because they were *not* willing to compromise.
"So fervently did Seward believe that war was unnecessary to destroy slavery that he made heroic but misleading efforts cultivate unionists in the Upper South in a desperate attempt to limit the scope of secession to the Deep South. Inevitably his behavior raised suspicions among the 'stiff-backs' in the Republican Party. Was Seward willing to offer the South a proposal that would allow slavery to expand into the territories? That was the line in the sand Republicans had drawn--no more expansion of slavery--yet only by abandoning that line was there any hope of avoiding war, if there was indeed any chance at all. Given Seward's desperate desire to avoid a war and his own driving ambition to take charge of the situation, some people thought he might cross the line and support an agreement that would allow slavery into some of the western territories. In fact he never did. Not once--in Senate debates, in public speeches, or in private correspondence--did Seward so much as hint that he would suport a settlement that would extend the Missouri Compromise line and allow slavery into the territories south of it. As Seward explained to his wife, he was trying 'to gain time for the new administration to organize and for the frenzy of passion to subside. I am doing this,' he added, 'without making any compromise whatever, by forebearance, concilation, magnanimity.'
"Whatever he was telling Upper South unionists in private, in public Seward conceded nothing. The tone of his January 12 speech to the Senate was conciliatory, his prose was obscure, but his substance was unyielding. There was nothing to distinguish it from the speeches that had marked him as a radical ten years earlier. He would not concede that slaves were 'property' under the Consittution, he would modify the Fugitive Slave Act of 1850, and he would suppress the southern rebellion by force. Secession was illegal and the Union indissoluble. In short, Seward held fast to the standard Republican position. But he did more than that.
"If it comes to war--if the North has to invade the South to enforce the law--the result, said Seward, would be slave rebellion. For although this was ostensibly a dispute over the disposition of the territories, it was at bottom a more fundamental conflict over 'the relation of African slaves to the domestic population of the country.' It was a struggle over slavery, and the slaves knew it, Seward warned. 'Freedom is to them, as to all mankind, the chief object of desire.' With a 'flagrant civil war' raging all about them, can anyone expect that the slaves 'will remain stupid and idle spectators'? Of course not, Seward argued. All of human history teaches us what the slaves will do in the midst of civil war. An 'uprising' and 'ferocious African slave population,' numbering in the millions, would overwhelm the South, and the entire slave system would come crashing down..."
(3) Even if Seward could not have prevented secession of the Deep South, could he have prevented civil war (and perhaps secession of the Upper South) by acquiescing in the secession of the Deep South states? Again, very doubtful. It is true that Seward was convinced that there would soon be a Unionist reaction in the Deep South, and for that reason tried desperately to avoid any violent incidents that could prevent such a reaction. He thus advocated abandonment of Fort Sumter. He also seemed to have the peculiar idea that the North and South could be united by a crisis--if not an actual war--with France or Spain.
https://groups.google.com/d/msg/soc.history.what-if/HfDoPtlOem0/gbVpY6q3OzsJ But this would not work, the expected Unionist reaction in the Deep South would not happen, and what would Seward do then?
I think it is absolutely impossible that he will acquiesce in disunion. He was far too much of an American nationalist for that. He wanted the Union not only to be preserved but to expand territorially. And besides, even if he had wanted to let the South go, it would be politically ruinous for him and for his party. The Democrats would blame the Republicans for breaking up the Union with their "abolition fanaticism." Republicans will also be angry at their party's course--"why did we let treason go unpunished and triumphant"? (Remember that a good many of them were under the delusion that a war with the South could be won quickly.)
So even if he lets *all* the forts in the South fall--there is some reason to believe that he really wanted to abandon Pickens as well as Sumter
http://www.tulane.edu/~sumter/Hesitation/Seward_adviceComm.html he has to do *something* to "enforce the laws" and indicate that the Union is still unbroken. Most likely this will take the form of an attempt to collect the revenues offshore. To the majority of Southerners this will be as much "coercion" and as much an act of war as resupplying Sumter. [1]
[1] As Thomas Clingman of North Carolina said:
"I confess, Mr. President, that I do not know whether or not I understand the views of [Buchanan's] message exactly on some points. There is something said in it about collecting the revenue. I fully agree with the President that there is no power or right in this government to attempt to coerce a State back into the Union; but if the State does secede, and thus becomes a foreign State, it seems to me equally clear that you have no right to collect taxes in it. It is not pretended that we can collect taxes at British or other foreign ports from commerce going in there. If a State of the Union secedes, and becomes a foreign State, it cannot be touched. The most offensive form of coercion which could be adopted would be that of levying tribute. I have no doubt that most of the governments of Europe would release their dependencies from the claim on them for protection and for postal facilities, &c., if they would just pay the government all the money it might think proper to exact. I do not know, sir, whether I am given to understand from the message that there is a purpose to continue the collection of duties in any contingency; but if that be the policy, I have no doubt some collision may occur."
http://books.google.com/books?id=ymUFAAAAQAAJ&pg=PA519