UPDATE 1-Probe of Cain's campaign finances sought

* Report alleges political group made illegal donations

* Cain aide worked for Tea Party aligned group (Updates with Cain quote on ties to Koch)

By Kim Dixon

WASHINGTON, Nov 4 (Reuters) - A consumer group asked U.S. regulators on Friday to probe allegations that Republican presidential candidate Herman Cain got illegal campaign cash from an influential conservative group backed by the wealthy brothers who run Koch Industries.

The Citizens for Responsibility and Ethics consumer group in Washington said it filed a complaint with the Federal Election Commission seeking a probe of a report alleging that tens of thousands of dollars in illegal contributions were made to Cain's campaign earlier this year by outside political groups.

Officials for the Cain campaign did not respond to a request for comment.

Corporations, including those that are not-for-profit, are not allowed to donate to presidential candidates. Only individuals and political action committees can give, and such contributions are limited.

The complaint comes as Cain is dogged by sexual harassment accusations by at least three women from when he was head of the National Restaurant Association in the mid-1990s. He maintains his innocence.

The charges against Cain could blunt his swift rise in the Republican race, where he has surged ahead of Mitt Romney or even with him in national polls and in Iowa, the state that holds the first U.S. nominating contest of 2012 on Jan. 3.

The illegal contributions complaint follows a report in the Milwaukee Journal Sentinel this week citing records of Wisconsin-based Prosperity USA and another political group that show they funded travel for Cain as he traveled in Iowa, the purchase of iPads, and other expenses.

The documents show that Prosperity USA, a unit of the national Tea Party-aligned group Americans for Prosperity, was owed about $40,000 by the Cain campaign. Such a payment would be illegal whether it was a donation or a loan to be paid back, the Citizens for Responsibility and Ethics said.

Cain campaign chief of staff Mark Block previously led Prosperity USA, and was affiliated with the second group.

David and Charles Koch, the billionaire brothers who run the oil and gas conglomerate Koch Industries, are the major financial backers behind Americans for Prosperity.

In remarks later on Friday to activists from Americans for Prosperity in Washington, Cain made fun of news reports that he had close ties to the Koch Brothers.

"I'm proud to know the Koch Brothers," said Cain. "I am the Koch Brothers' brother from another mother."

Officials at Koch Industries were not available for comment, and Americans for Prosperity declined to comment.

With the limited information available, several campaign finance experts said there is potential that several laws were broken.

"There are certainly red flags all over the place," said Paul Ryan, a lawyer at the Campaign Legal Center. (Additional reporting by Eric Johnson; editing by Philip Barbara and Doina Chiacu)