Dave Leip's Atlas of U.S. Presidential Elections is the work of a single individual. According to the author, it was started as a paper hobby after the 1992 Presidential Election. Dave Leip uses Excel as a tool to produce the pie charts and to help build the county-level maps.
There are two distinct elements:
Important note: Dave Leip's election maps use RED for the Democratic Party and BLUE for the Republican Party. See his FAQ for more information.
UO Libraries has purchased a number of data spreadsheets from Dave Leip, with Presidential, Senate, and Representative election returns, and voter turnout & registration data, down to the county level.
Important information about accessing these:
If these links do not work for you, go directly to Oregon Digital and login as a UO user. Do a search on the word leip, and you will find all datasets available. You can browse or search within the Leip datasets to find the year you are interested in, etc.