The Washington Post has collected (links to) over 500 ads in the database, and this is the description of the resources:
The database includes political advertisements funded by campaigns, parties, committees, and independent advocacy groups. Most of the ads are tied to specific U.S. House, U.S. Senate, or gubernatorial races throughout the country. Some of the ads are more general "issue" or advocacy ads not tied to a particular race or candidate. You can search for ads based on the criteria listed below.They are categorized by campaign, person, content, tone, characters, etc. Some of you may be especially interested in this group of emotional ads.
It's a nice idea in theory, but of the 5 ads I tried to watch, only 2 worked. Still, if you're interested in politics & communication, you might want to check this out.