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WASHINGTON (Reuters) - A Democratic U.S. senator said on Thursday she will ask Facebook Inc FB.O and the Justice Department about a media report that the company hired an outside firm to attack critics, warning that such action could raise campaign finance issues.

Senator Amy Klobuchar told the Senate Judiciary Committee she would send a letter seeking details about a New York Times article that named the Minnesota Democrat as a target of an aggressive Facebook lobbying campaign that relied partly on a Republican opposition research firm.

Facebook did not immediately respond to a request for comment on Klobuchar. But Facebook Chief Executive Mark Zuckerberg told reporters that he ended the company’s relationship with the outside firm, Definers Public Affairs, after he became aware of it while reading the New York Times story. Definers did not respond to a request to comment.

The New York Times said Facebook conducted an aggressive campaign to combat critics, shift public anger toward rival companies and fend off new regulations contained in proposed legislation that Klobuchar supports.

“This is a pretty serious matter for those of us on the receiving line,” Klobuchar told the committee.

“I would like to know what was going on with elected officials,” she said. “If they in fact were taking actions against critics, this could be a campaign finance issue... It could also have other legal ramifications.”

If Facebook spent money trying to influence the outcome of an election, it would have been required to disclose that spending to the Federal Election Commission and failure to do so would be a violation of the law. However, recent Supreme Court rulings have given companies leeway, requiring that they directly call for someone to be elected, or lose their office, to trigger the campaign finance laws.

The Times report said Facebook hired the Republican firm to discredit activist protesters and lobbied a Jewish civil rights group to cast some critics of the company as anti-Semitic.

Its issue with Klobuchar involved legislation known as the Honest Ads Act, which she has supported along with Democratic Senator Mark Warner and the late Arizona Republican Senator John McCain. The bill would compel Facebook and other internet firms to disclose the sponsors of political ads on their websites.