That's easy... Jefferson Davis.
The problem there is that had Jeff Davis been tried and found guilty of treason, the same charge could have been brought against Lincoln. Since Article III, Section 3 of the U.S. Constitution provides that New States may be admitted by the Congress into this Union; but no new State shall be formed or erected within the Jurisdiction of any other State.... the creation of the state of West Virginia following the secession [peacefully, BTW] of 55 counties of the Confederate State of Virginia as the State of Kanawha, thence to be admitted into the Union as *West Virginia.*
Since Lincoln's action in admitting West Virginia as a state constituted a de facto recognition of the lawful secession of the Confederate state of Virginia, his actions constituted an act that gave comfort and a certain legitimacy to the Confederate cause, and was certainly grounds for his impeachment; John Wilkes Booth and his friends saved him from that possibility.
Impossibility, more like. Western Virginia didn't secede from anything. The recognized legislature of Virginia voted to split and Congress voted to allow them.