Imagine a congressional district where one party controlled the voter lists, the jobs and working conditions of all the voters, and held the power to determine whether or not their children were going to have the money for college, a dental visit, or even their parents home from work at a decent hour.  

Imagine this same congressional district, in which the other party's organizers had little access to the voters or even an address list that told you where the registered voters lived, let alone a list of the potential voters.  

Imagine that these organizers and the voters they tried to mobilize could be fired at will, have their home and family disrupted, and have their pay and benefits cut.  Those voters willing to go along with the party in charge can legally be bought off with small perks - at least until the challengers have given up or lost their jobs.

And if, by some chance, the challengers actually win the election, imagine the defeated incumbent getting to refuse to step down and forcing the challengers to initiate years of litigation to actually take office in an election they won....