MINNEAPOLIS  — The U.S. attorney’s office is suing Sen. Sean Nienow and his wife, alleging they failed to make payments on a Small Business Administration loan.

The lawsuit, filed Friday in Minneapolis, claims the Republican lawmaker and his wife, Cynthia, stopped making payments on the $613,000 loan in July 2010.

The suit alleges they owe nearly $748,000 with costs. A message left Saturday at his home in Cambridge wasn’t immediately returned.

The couple borrowed the money in 2009 for their former company, the National Camp Association Inc., which was described in media accounts as an organization that helps parents select a camp.

The association, which is listed on the Minnesota secretary of state’s website at the Nienows’ address, also was sued.

The website shows that the association was incorporated in 2008 and dissolved in 2012. Nienow was described as the chief executive officer.

Nienow, who is considered a fiscal conservative, served in the state Senate from 2003 through 2006. After being defeated for re-election, he won back the seat in 2010, and won again in 2012. In the official directory of the state Legislature, the 46-year-old is described as a “consultant” with no additional details.

A copy of a summons filed in the federal lawsuit states that the Nienows must answer the complaint against them within 21 days after the suit is served.