That page you linked (http://memory.loc.gov/ll/llfr/002/0000/00610057.tif http://memory.loc.gov/ll/llfr/002/0000/00610057.tif) contains that argument from Madis

That page you linked (http://memory.loc.gov/ll/llfr/002/0000/00610057.tif http://memory.loc.gov/ll/llfr/002/0000/00610057.tif) contains that argument from Madison followed by a non-slavery based argument made by someone else. To say that the electoral college exists "because of slavery" is an oversimplification. It ignores the misgivings about a popular vote that are mentioned in the page you linked, and it also ignores the desire of the state governments to exert control over their representation in the Federal government -- senators were not directly elected for the same reason. If you want to point to part of the Constitution that exists purely because of slavery, the 3/5 compromise is a better example. The decision to adopt an electoral college was certainly influenced by complications introduced by slavery, but there were a lot of other arguments in favor of an electoral college that were persuasive at the time.