“I don’t agree, I think a united Democrat candidate in 1860 had a fighting chance.”
That’s not what the electoral numbers say. Go, look up the birth state of presidents from Grant onwards and you’ll see the point.
Also, regional politics played an important role. Lincoln checked Douglas in his own region and handily carried the Northeast.
If Seward had been nominated, a Democrat could have picked up votes in the Old Northwest. With Southern votes such a Democrat could have won. Conversely, a New Yorker like Seymour just possibly could have beaten Lincoln.
Please consider this:
In 1856, "Dough-Faced" Northern Democrat James Buchanan was elected president by winning Pennsylvania, Indiana, Illinois and California, plus every Southern state.
He immediately went to work to support the Supreme Court's Dred Scott decision.
Just one more election like 1856, and one more Dred Scott type Supreme Court decision would have effectively made slavery legal in all states.
That's how close the slave-power came to victory.
But in 1860, many people who had voted for Democrats all their lives switched to other parties, because the majority Democrats had committed political suicide.
I'm saying a united, enthusiastic Democrat party in 1860 had a fighting chance to keep those "swing voters" in the Democrat fold, and win the election by carrying those same states it won in 1856.