ABC News’ Huma Khan and Emily Friedman report:

In a vivacious speech this afternoon, Herman Cain defended his ties to the Koch brothers, the billionaire oilmen famous for bankrolling conservative campaigns.

“I am the Koch brothers’ brother from another mother … and proud of it,” Cain said at a summit in Washington, D.C., today held by Americans for Prosperity, a group founded by David Koch.

That statement spurred a loud cheer from the audience.

Cain also chided the media for exaggerating their relationship. The Republican candidate was hired by Americans for Prosperity, a conservative non-profit advocacy group, in 2005 to promote the group’s agenda of cutting taxes and government regulation, and dampening the influence of unions, according to a story by the New York Times, published Thursday.

Cain’s chief of staff, Mark Block, ran the group’s Wisconsin chapter, and is now under scrutiny for allegedly having made illegal contributions to Cain’s campaign through his own organization, Prosperity USA. Citizens for Responsibility and Ethics in Washington, a watchdog group, today filed a complaint with the Federal Election Commission based on those allegations, which were first reported by the Milwaukee Journal Sentinel.

“I’m proud to know the Koch Brothers, I’m very proud… They make it sound like that we have had time to go fishing together, hunting together, skiing together, golfing together,” Cain said, referring to the New York Times’ article.

As Cain spoke, David Koch – the founder of the energy company Koch Industries – cheered from the front row.

Last month, Cain told CNN that he doesn’t have a close relationship with the Koch brothers, but they all know and respect each other.

Cain joked today that he was in Washington all week garnering a “little bit of attention,” referring to the multiple allegations of sexual harassment that have surfaced this week.

The lawyer for one of the accusers contradicted Cain’s claim that he was falsely accused, saying that while his client does not want to publicly tell her story, she maintains that she was the subject of inappropriate actions by Cain.  The accuser, a woman who now works for the federal government, was the subject of sexual harassment more than once when Cain headed the National Restaurant Association in 1999, her lawyer said this afternoon.

Cain was one of the two keynote speakers at today’s event. His rival Mitt Romney, the first speaker, elicited applause from the crowd but not quite as much as Cain.

As the former chief executive of Godfather’s Pizza walked in, people stood up, cheered and clapped and screamed. Some even rushed to the stage to get photographs. Romney only got one standing ovation, when he said he would repeal the federal health care law.