Apparently IOTL the Maine delegation in the 1860 Republican convention (supported by our good friend James G. Blaine) tried to start a movement to nominate Senator William Pitt Fessenden, a former Whig and prominent anti-slavery Republican in the Senate. It died down when Fessenden explicitly forbade them to nominate them. Let's say he's more open to the idea and ends up being nominated - what does a Fessenden presidency during the Civil War look like?