A political nonprofit group founded by the billionaire industrialists David and Charles Koch spent $122 million during 2012, according to documents filed with the Colorado secretary of state, putting it among the biggest election spenders in the country.

The group, Americans for Prosperity, spent tens of millions of dollars in political advertising in a bid to defeat President Obama and Democrats in Congress, while investing equally large sums to build a national grass-roots organization for conservative activists and voters. Based in Virginia, the group is central to the political organization nurtured by the Koch brothers and their allies in the past decade, and in recent years has played a pivotal role in advancing conservative policies in states like Wisconsin and North Carolina.

But the group and other right-leaning nonprofits and “super PACs” came up short last year, failing to unseat Mr. Obama or swing the Senate to Republican control. In recent months, the Kochs have undertaken a major revamping of their political network, including building a more centralized fund-raising apparatus centered on a trade association called Freedom Partners, also based in Virginia.

Because Americans for Prosperity is not required to disclose its donors, it is unclear how much of the group’s money comes directly from the Kochs or other donors. And some comes from other nonprofits, like Freedom Partners, which provided $32.3 million to Americans for Prosperity in the months leading up to the 2012 elections.