Proponents of the Supreme Court’s Citizens United ruling have argued  that Super PACs help shake up the almost-guaranteed reelection prospects for congressional incumbents, going so far as to cast Super PACS as a way to bring about a new era of good government. But the very nature of the Super PAC – a mechanism that enables wealthy individuals and special interests to secretly funnel money through corporate political donations in support of their favored candidates – practically invites corruption.
From the New York Times :
Incumbents in Congress usually have a huge fund-raising advantage over challengers. Big donors correctly assume they will probably be in office for years, and curry favor with contributions that only wealthy challengers can match. So why not try to neutralize this advantage by spending money on behalf of challengers? …
But the method they are using — a super PAC that can collect and spend unlimited amounts of money — is the opposite of good government, and demonstrates the inherent danger in allowing big money to steer election results. The handful of donors say their motives are pure, but the public has no way of knowing what their long-term goals are, or whether they have personal interests in the races they have chosen.
The electoral advantages incumbents enjoy is indeed a problem, but not one that is caused or solved by Citizens United. Super PAC funds are comprised not by grassroots donations but by large contributions from a few  wealthy donors. Using these resources to usher challengers into office perpetuates the fundamentally anti-democratic influence of special interests of money in our elections.
Regardless of whether wealthy special interests seek to reelect reliable incumbents or replace them with sympathetic challengers, the end result is that the most influential voice in our elections is not that of the American people. We need a constitutional amendment to overturn Citizens United in order to level the playing field for everyone – wealthy or otherwise.