I recommend you read the full article The Great Gerrymander by Prof. Sam Wang [http://www.nytimes.com/...]
Wang adds up all the votes for Democrats and Republicans in a state's various Congressional districts and compares the statewide party percentages with the proportion of Congressional seats which go to each party to see the impact of gerrymandering. Wang says, "start with the naïve standard that the party that wins more than half the votes should get at least half the seats. In November, five states failed to clear even this low bar..."
"In North Carolina, where the two-party House vote was 51 percent Democratic, 49 percent Republican". But the state's elected Congressional delegation was, "Four Democrats, nine Republicans". 51% of the votes resulted in 31% of the seats!
"In the seven states where Republicans redrew the districts, 16.7 million votes were cast for Republicans and 16.4 million votes were cast for Democrats. This elected 73 Republicans and 34 Democrats." In other words, Democrats got 49% of the vote [at least excluding votes for third parties] and got (almost) 32% of the Congressional seats.
In all, the Republicans made more of a difference for themselves in the 2012 election than Democratic gerrymandering, but both parties do gerrymandering and other tricks. My first full-time job after college was in a city run by a Democratic political machine comparable to old Chicago. That's probably part of the reason why we don't hear the Democratic Party making a bigger stink about what the GOP has done. Still, regardless of whether the Democrats were motivated by civic desire to maintain majority rule or motivated by cynical desire to maximize the number of their own people in office, this should be a critical issue for the party.
There are a number of other mechanisms limiting true majority rule in the US. There's the Electoral College in presidential elections, there are attempts at voter suppression or intimidation, about 1% of the voting age population are former felons (no longer in jail, probation or parole) denied the right to vote (see: http://www.dailykos.com/...), etc. Unlimited spending by the rich to influence elections is also a critical issue.
Fair elections, fair districting, fuller freedom to vote and similar guarantees on democracy should be a top priority. It should be a make-or-break question for any candidates you support and it should be a major effort between election seasons.