The rising clout of independent political players such as super PACs, advocacy organizations and even for-profit corporations has led to what some describe as a shadow political system. Unencumbered by fundraising limits, these groups are increasingly taking on functions that traditionally were the domain of the political parties. Here are some of the most influential outside groups.

On the Democratic side:

Organizing for Action

President Obama. (Haraz N. Ghanbari/AP)

Function: Grassroots organizing

Who is behind it:: Launched in January as an offshoot of President Obama's campaign organization, OFA is a nonprofit advocacy group ran by former top Obama advisers to promote his second-term agenda.

Patriot Majority USA & Senate Majority PAC

Function: Paid media on behalf of Democratic Senate candidates

Who is behind it: This nonprofit and super PAC, run by allies of Senate Majority Leader Harry Reid (Nev.), often combine forces on TV campaigns taking on GOP senators.

House Majority PAC

Function: Paid media on behalf of Democratic House candidates

Who is behind it: This super PAC operates as the primary bulwark for Democratic House candidates, guided by a former Democratic Congressional Campaign Committee strategist.

American Bridge 21st Century

Function: Opposition research and rapid response

Who is behind it: A super PAC with a nonprofit arm, it serves as a research clearinghouse for the congressional super PACs and other groups on the left. Launched by David Brock, who also started Media Matters for America, the liberal media watchdog group, it is run by Rodell Mollineau, a former Reid staffer.

On the Republican side:

American Crossroads & Crossroads GPS

Function: Paid media, voter outreach and research

Who is behind it: Co-founded by veteran party strategist Karl Rove and Steven Law, a former chief of staff to Senate Minority Leader Mitch McConnell, the Crossroads organizations were some of the biggest players in the 2010 and 2012 elections.

Americans for Prosperity

Function: Grassroots mobilization, paid media

Who is behind it: The nonprofit advocacy group, which has received backing from Charles and David Koch, has spent heavily on ads critical of the Obama administration.

Congressional Leadership Fund

Function: Paid media aimed at protecting the GOP's House majority

Who is behind it: House Speaker John Boehner (Ohio) has helped raise money for this super PAC, run by veteran party strategists.

America Rising

Function: Opposition research and rapid response

Who is behind it: Launched this spring by former Mitt Romney campaign manager Matt Rhoades, RNC research director Joe Pounder and RNC spokesman Tim Miller, this venture – set up as a LLC and a super PAC – plans to research and track Democratic candidates.