What's up for grabs in 2014? Try 435 seats in the House, one-third of the 100-member Senate and 36 governor's offices.
Outside spending will play a pivotal, if not deciding, role.
This year's elections will be the first when both parties fully embrace outside groups and their potential to accept unlimited contributions from supporters.
The usual players such as the National Republican Senatorial Campaign and the Democratic National Committee are expected to keep helping candidates. But other major groups on the outside also are now poised to influence the races.
—American Crossroads and Crossroads GPS. The partner organizations take their cues from strategists Karl Rove and Carl Forti, as well as former Republican National Committee heads Haley Barbour, Ed Gillespie and Mike Duncan. Considered the heaviest hitter among Republican outside groups, the two spent $176 million in 2012, mostly criticizing Democrats. Only donors for American Crossroads, a super political action committee, are disclosed.
—Americans for Prosperity. The chief outlet for libertarian billionaire brothers Charles and David Koch. The group helped the nascent tea party rise in 2010 and has spent heavily in support of libertarian-minded Republicans. Tax filings in Colorado from 2013 showed the group spent $122 million during 2012 but came up short in its bid to defeat President Barack Obama. Because of the way it is structured, it does not have to disclose its donors.
—Heritage Foundation. The longtime think tank on Capitol Hill has stepped up its political machinery since former Sen. Jim DeMint, R-S.C., took over. The group's political arm, Heritage Action, played an outsized role in October's government shutdown, warning lawmakers who supported a compromise that there would be consequences. The 2014 elections will be the first ones with DeMint calling the shots.
—U.S. Chamber of Commerce. The biggest business coalition with deep pockets for Republicans. In 2012, the group spent almost $36 million to help GOP candidates and causes. The establishment-minded group has taken sides during some Republican primaries, coming to the aid of more mainstream candidates. It does not disclose its donors.
—America Rising. A research shop that is run by GOP operatives, including Mitt Romney's campaign manager, Matt Rhoades, and RNC aides Joe Pounder and Tim Miller. The group and its affiliated consulting firm send staffers to track Democratic candidates with the goal of capturing their gaffes. The researchers also dig into Democrats' histories. America Rising then shares information with fellow conservatives.
—FreedomWorks. Another Koch-backed operation, this one focused on mobilizing rank-and-file conservatives to support conservative Republicans running for office. The group trains grassroots activists to fight for lower taxes and less government and threatens Republicans if they break from party orthodoxy. It took its name from a group started by former House Republican Majority Leader Dick Armey of Texas, who is no longer part of the organization.
—Club for Growth. A take-no-prisoners enforcer for candidates who pledge to lower taxes. The well-funded group spent $17 million to help candidates, and punish defectors, in 2012 and has sketched out a strategy to go after incumbent Republicans who stray from the group's line. Its current leader is former Indiana Rep. Chris Chocola, who took over when Pat Toomey was elected to the Senate.