By one measure, next month's Upper House election is probably Japan's most important national poll ever.

The race, which kicked off Wednesday, could pave the way for Prime Minister Shinzo Abe to make history: to revise the U.S.-drafted Constitution nationalists see as a humiliating remnant of Japan's World War II defeat. Since its inception nearly 70 years ago, the national charter has not been tweaked once.

Although Abe's ruling Liberal Democratic Party and its junior partner, Komeito, already hold a two-thirds majority in the 475-seat Lower House, they have yet to achieve this in the 242-seat Upper House. Abe wants to change this by winning big in the July 10 election, allowing pro-amendment forces to control two-thirds in both chambers — a scenario that would give him leverage to call a national referendum on constitutional revisions.