Much as you may sympathize with the City of Detroit, and admire Miguel Cabrera and Justin Verlander, if you're a Democrat, you need to root against the Tigers in the Series.
Why? Well, the Tigers have won the Series in two Presidential years - 1968 and 1984. Not good years for our team. '68 is an especially scary precedent: the Tigers beat the St. Louis Cardinals and a shapeshifting Republican with no actual core beliefs won the Presidency.
The Cardinals, by contrast, bring Democratic Presidents in their Series-winning wake. They won in 1944 and 1964. As an added bonus, when they beat the Tigers in '06, they swept in a Democratic House.
The Giants have never won a World Series in a Presidential year. They've lost three (1936, 1912, 1904), and Democrats won in two of those years with Teddy Roosevelt winning the third (actually, the first). But I wouldn't want to bet on "good things happen when the Giants lose the Series"; the series winner is clearly the history-shaping actor, which in this case would be the bad-luck Tigers.
If the Giants win, there's an argument that it would be good luck for Democrats because of their effect on House races: Giants Series victories are correlated with historic gains for the out-party. In 1922, the Democrats won over 70 seats; in 1954 they recaptured control; and in 2010 the Republicans gained control.
Now, I know that some of you out there are thinking that I'm putting too much stock in small sample sizes. You are wrong. Remember, it's not sample NUMBER that matters; the phrase is sample SIZE. A World Series, any World Series, is an enormous event. Therefore, even one World Series is a large sample size - let alone several.