In the coming year or so states will begin to outline plans for the redistricting process that will go into effect in 2012 after the 2010 Census results. One way to measure the 'success' of these processes is to examine to what extent the proportion of votes cast for a particular party translates into the proportion of seats won in government.
Smart Politics analyzed US House elections in the Upper Midwest since 1960 and found—collectively—the proportion of votes cast in Minnesota and Wisconsin between democrats and republicans bore a close relationship to the number of seats won between these two parties, while the number of seats won in Iowa was heavily skewed towards the Republican Party.
In Minnesota, the DFL / GOP split 54% to 46% respectively in votes cast since 1960 with the DFL having a 57% to 43% advantage in seats won during this span (105 to 80 seats).
In Wisconsin, the relationship is very similar: democrats and republicans are split 50% to 50% among the nearly 40 million votes cast for those parties since 1960, with Democrats winning a narrow majority of seats (109 to 102, or 52% to 48%).
In Iowa, the distribution is much more skewed in favor of the republicans. Republicans have a 52% to 48% advantage in votes cast, but a much larger 64% to 36% advantage in seats won (88 to 50). For example, in 1990, the GOP won 4 of the 5 US House seats, but there were actually more votes for democratic candidates (400,852) than republican candidates (385,003).
(In South Dakota, republicans have a 51.5% to 48.5% advantage in votes cast, but a 62% to 38% advantage in seats won (21 to 13). South Dakota is a special case, however, because since 1982 it only has an at-large seat, so there is only one district in the race for US House).