The nonprofit, known as American Future Fund, has said it will spend $250,000 on ads supporting Martin in Missouri.
The group launched its first ad this week.
American Future Fund is based in Des Moines, Iowa. Over the years it has spent millions of dollars trying to elect Republicans around the country, mostly on congressional races. Its leadership is made up of prominent Iowa Republicans with ties to the failed 2008 presidential campaign of Mitt Romney, as well as media consultants who played key roles the Swift Boat Veterans for Truth ads in 2004 and the Willie Horton ad in 1988, both of which targeted Democratic presidential hopefuls.
In 2010, after it spent nearly $1 million trying to unseat Democratic U.S. Rep. Bruce Braley, it was discovered the American Future Fund's headquarters was a P.O.Box at a UPS Store in Des Moines.
In addition to the ad buy from American Future Fund, U.S. Sen. Marco Rubio, a Florida Republican, held a fundraiser with Martin in the St. Louis area earlier this week. On Monday, Texas Gov. Rick Perry will be in Columbia for a business roundtable and meet and greet with Martin.
Perry has campaigned in Missouri for Martin this summer in Cape Girardeau.
Rubio said he decided to support Martin because of his promise to fight the federal health care law, and idea that Martin has made the centerpiece of his campaign.
The last campaign disclosure reports filed with the Missouri Ethics Commission show Koster, a Democrat, with a massive fundraising lead.