Philip Bump has this list of instances when one major party, or its adherents, tried to determine the nominee of the other major party, via advertising. The examples cover the last twelve years, and are mostly gubernatorial and U.S. Senate races. Bump also includes one U.S. House race, and one presidential race, the Democratic contest in 2004. There are many other recent examples involving state legislative races and U.S. House races, especially in California and Washington. Thanks to Rick Hasen for the link.