Good post!
I'm still not certain I want to agree that a "lock" even existed since the 1860 start of the democrat- republican two party system. Before that I cannot say with certainty.
As to Demrepdans suggestion that a republican lock existed from 1861 to 1885 I would say that certainly the country was tilted towards them but I am uncertain of a lock (which would mean democrats had no chance at all)
The 1868 election is indicative of this. It was not certain from the start and could well have been much closer than is generally believed. Grant won by carrying several southern states because he received nearly 100% of the black vote and many white voters did not participate. (WOW!!! those were different times huh?
It is widely believed that he lost the white vote to Sam Tilden and while a lot of people voted for him because of his war record, their vote was not certain from the start but the war record was the deciding factor at the end. The result of this election was by no means certain from the start.
Note: Obviously there was no polling at the time and this is based on contemporary records.