Which party will win the popular vote in the U.S. Presidential Election?
If this market is created, it seems to me that it would generate more interest as a "Popular Vote Margin" contract series. If the polls strongly favor either party, then the margin of victory will remain interesting into the Fall. The "which party" market, on the other hand, might slow down a lot if it's 85/15 or 90/10 for either party.
Since there is already a market on the election winner, are you really asking for a contract on whether the popular vote winner will differ from the Electoral College winner? That could be very interesting if polls (and the election) are very close. I'm not sure how to make it interesting if polls are projecting a landslide, though. A market on (Popular Vote Margin - Electoral College Margin) seems pretty esoteric.