Is there a database of which party (Democrats/Republicans/Other) won which county/zip code in a U.S. presidential election? Even better would be a panel of these data. If no public database like this exists, which I find hard to imagine, what do you think the best way of getting these data would be?
The open elections project is trying to collect this type of information from official sources, though it isn't yet available.
This is commercially valuable information that typically comes with a licensing fee and can be problematic for republishing. "Election Data Services" is one of the usual sources for this data. Also see Dave Leip's political atlas store where a site license for 2012 county-level presidential results appears to be about $200.
This data doesn't exist for ZIP codes, of course; they don't match polling districts.
A word of caution about using the AP data made available through client sites such as Politico (or, for that matter, the New York Times): AP does make updates to its election results data, but most news organizations do not update their published data more than a few days after the election. So in most cases what you'll find in such feeds is incomplete data for some states and counties. This is one reason why we're doing OpenElections, and we will have county-level presidential results.
I know that a number of the news organizations have made graphics showing visualizations with basic party-by-county vote information, so I'm fairly certain it's out there ... but I don't know if it's open in its aggregated form.
Some newspapers have a sort of co-op that they call the National Election Pool to get presidential election results ... but again, it doesn't mean that the data's available to others in its raw form.
The good news is that even if you had to collect it up yourself, I would assume that there are enough other non-profits and similar interested groups (eg, Project VoteSmart, League of Women Voters, Center for Responsive Politics, Center for Public Integrity, Sunlight Foundation, etc) that you could probably make your own co-op. You might also have luck attempting to contact state Board of Elections, and attempting to determine if they have county results, rather than having to contact each local election board individually. (and/or attempting to convince the state board that they should become an aggregator of this information)
There's nothing completely pre-packaged that I'm aware of, but this page has a lot of decent resources, including recent county-level presidential voting data, but only under a commercial license. The gold standard for comprehensive data is CQ Press's Elections collection, but unfortunately it's commercial.
I know some folks have used the Associated Press's data for visualizations of the last election through links like this:
It's actually a semi-colon separated values result, not XML, and you'll have to crawl for each state (replacing NV in the URL with the other state codes). There's no license information, but it has been used for academic work.
In Louisiana election results for all races are available on the Secretary of State's website down to precinct level, as well at the Parish (Louisiana's equivalent for counties) level, though you will have to scrape the web pages to gather the data. Here is one example page at parish level: http://staticresults.sos.la.gov/11062012/11062012_17.html, which includes link to access precinct level data. Other states may have similar data on state's Secretary of State web sites since those offices generally are responsible for election returns.