"There is not a day to lose in disavowing emancipation or Kentucky is gone over the mill dam -"
This appears to be referring to the need for Lincoln and the Republican Party to proclaim that they did not plan to emancipate the slaves of the South and Border slave states in order to keep Kentucky from seceding. Was this a threat? It was sent on September 13, 1861, just days before Kentucky's brief attempt at neutrality ended. Lincoln had clearly stated in his first inaugural address that previous March that, "So, I am not so sure. Maybe they wanted another confirmation. It's not real clear to me.
I do know that Bullitt was later arrested by order of the Lincoln administration for conspiring to overthrow the government, so possibly it was a threat.
If anyone has any ideas, I'd be interested to hear your thoughts?