it's actually just the areas where republicans are contesting. they are known for playing a dirtier/smarter game than the democrats, who act like pussies compared to them.
Are you smoking weed / shrooms right now? The vote rigging mostly happens in the inner city areas, where the Democrats get close to 100% of the votes. There are plenty of examples.
not that it matters, but data analysis from one individual (Sam Wang, a neuroscientist from Princeton) suggests otherwise
http://election.princeton.edu/2012/12/30/gerrymanders-part-1-busting-the-both-sides-do-it-myth/There are some simple lessons to take away from this.
Republican-controlled redistricting led to a swing in margin of at least* 26 seats, almost as large as the 31-seat majority of the new Congress. Those actions created a new power reality in the House – or more accurately, retained the old power reality.
In the states listed above, the net effect of both parties’ redistricting combined was R+11.5 seats. Putting all of this redistricting into nonpartisan commissions would lead to a swing of at least 23 seats. The resulting seat count would be 213 D, 222 R or even closer. It is possible that in the absence of partisan gerrymandering, control would have been within reach for the Democrats.
I do not know of the slant of the academic at hand (he does a pretty good job of sounding neutral), though many in academia have leftist slants, in that academia often depends on public funding for research.
edit: definitely leftist, lists Satan/Krugman in a "blogroll" reading list on the left of his website
edit 2: bryant.coleman, your claim is supported with at least Illinois's data