"I don't know how a congressman who claims to be transparent doesn't disclose this beforehand," said real estate investor Wil Cardon, Flake's opponent in the GOP primary Aug. 28. "It makes me question every vote he has taken when it comes to Iran."
Cardon added: "It's also confusing to me that Flake has taken tens of thousands of dollars from the mining industry. From a guy who was a lobbyist before going to Washington, and claiming to fight special interests, it's interesting how beholden to special interests he has become."
Since joining Congress, Flake has received at least $97,000 from mining interests, including Rio Tinto, according to the Center for Responsive Politics, which tracked donations through the 2010 election cycle.
He has backed legislation favorable to mining interests in Arizona. Last year Flake proposed ending a ban on uranium mining claims around the Grand Canyon. In 2009 he introduced a land swap bill that would enable Resolution Copper, an arm of Rio Tinto, to develop a mine in eastern Arizona.
Democrats also are making Flake's lobbying past an issue.
"Jeff Flake is the definition of a Washington D.C. insider," said Matt Canter, a spokesman for the Democratic Senatorial Campaign Committee.
"He spent years as a Washington lobbyist and a registered foreign agent, doing the bidding of special interest foreign clients, before becoming a career politician who voted to benefit his former clients and refused to tell his constituents about his conflict of interest," Cantor said.
Flake's campaign denies any conflicts and says his critics are twisting facts from a background that he's never tried to hide.
"I have mentioned it in bios, I have mentioned it in numerous speeches. So this notion that I tried to hide my past is farcical," Flake said in a recent interview.
He says his record as one of a few congressmen who have repeatedly voted against sanctions for Iran is irrelevant. He says he long has opposed all such unilateral penalties, whether against Iran, Sudan, Cuba, Myanmar or any country.
That opposition, he said, dates back to his time in Namibia, where he saw the damage that sanctions can have on innocent people.
Flake's biography: http://flake.house.gov/Biography/
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