State Sen. Sean R. Nienow, R-Cambridge, and his wife, Cynthia, have been sued and are accused of failing to make payments for the past 3½ years on a $613,000 loan they took out from the Small Business Administration (SBA).

The allegations are contained in a lawsuit filed Friday afternoon by the U.S. attorney's office in ­Minneapolis.

The Nienows borrowed the money for their company, the National Camp Association, Inc., described in news 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' home address in ­Cambridge, 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.

The suit alleges that on Jan. 16, 2009, the SBA provided the loan to the association and it was personally guaranteed by the Nienows. A copy of the signed guarantee is attached to the suit.

"The defendants ceased making payments on or about July 28, 2010, and the Note has not been repaid," the suit said. "Defendants have defaulted under the terms and conditions of the Note by failing to repay said Note."

Although demand for ­payment was made, the Nienows did not comply, the suit said, and they owe an unpaid principal balance of $558,076.53 and administrative costs of $189,861.09 for a total of $747,937.62.

Sen. Nienow, 46, did not respond Friday to phone calls, a text message or e-mail.

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, he is described as a "consultant" with no additional details.

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

Randy Furst • 612-673-4224