The super PAC was required to make the report because of its involvement in South Carolina’s 1st District special election backing the Democratic candidate Elizabeth Colbert Busch. It has already spent nearly $150,000 to support Colbert Busch and attack Republican candidate Mark Sanford.

There were few surprises in the House Majority PAC filing. Major donations included $200,000 from Jon Stryker, an heir to the Stryker Corporation fortune and an LGBT activist, and $100,000 from hedge fund head Don Sussman, who is married to Rep. Chellie Pingree (D-Maine). Unions chipped in, including $50,000 from the United Auto Workers, $35,758 from the PAC of the Service Employees International Union and $30,000 from United Steelworkers PAC. The only corporate contribution came from Klein Financial Corporation, which is run by Bob Klein, a noted Democratic donor and supporter of stem cell research.

House Majority PAC also raised money from 1,800 small donors with an average contribution of just over $20. These donations, however, accounted for only a small slice of its overall fundraising.

The super PAC took in a large chunk of its early 2013 money in refunds related to last year’s spending. In particular, it received $325,000 from Waterfront Strategies, a media buying arm of the Democratic consulting firm GMMB. This was money refunded for advertising that ended up not running in the 2012 election cycle, according to a House Majority PAC spokesman.

In the 2012 election, House Majority PAC spent more than $30 million on congressional races. It was eighth in spending among non-party independent groups in the election and won more than 70 percent of the races in which it participated, according to the Sunlight Foundation.

House Majority PAC disclosed now because of its involvement in the Sanford-Colbert Busch battle. Other super PACs will not file reports until July due to FEC rules that allow independent groups and PACs to choose between a schedule of monthly or quarterly disclosure. Those that choose to disclose quarterly are only required to file twice in non-election years. Nearly every super PAC changed its status from monthly to quarterly at the conclusion of the 2012 election.