Bruce Braley, a Democrat from northeastern Iowa, has been a popular two-term congressman and seemed likely to have an easy re-election until the huge cash mudslide of 2010. The Republican Party had largely left him alone, but then a secretive group called the American Future Fund began spending hundreds of thousands of dollars on distortion-heavy attack ads.

Mr. Braley is now struggling to maintain his lead against a Republican challenger, Benjamin Lange, who is running on a familiar program of smaller government and opposition to the health care law, the stimulus and growing federal spending. Mr. Braley has disclosed all of the donors behind his ads and his campaign; Mr. Lange generally will not discuss his independent support.

Mr. Braley has shown admirable political courage throughout the race, staunchly defending his support for health care reform, the stimulus and the Bush-era bank bailout. Each will benefit the country over time, he said. “I’m going to stand my ground and won’t be intimidated,” he told a local radio station a few weeks ago.

That position stood him well in the relatively liberal 1st District of Iowa until he became a target of the American Future Fund, one of several conservative groups spending millions of dollars to defeat Democrats while promising their donors anonymity.