Veteran Democrat Ed Markey, who replaced Secretary of State John Kerry last year as the junior senator from Massachusetts, is being accused of using his position to manipulate stock.
A Republican politician, Brian Herr, who plans to challenge Markey in this year's election, issued a statement
on Monday saying he has filed a complaint against Markey with the Senate ethics committee.
Markey has been under fire for writing letters to federal regulators urging an investigation into the nutritional supplement firm Herbalife.
Critics such as Boston Globe columnist Joan Vennochi have charged
that he developed his interest in the company after his staff met with a hedge fund tycoon, William Ackman, who is a prominent donor to the Democratic Senatorial Campaign Committee.
Ackman has taken a $1 billion investment in Herbalife stock called a short — essentially a bet that will pay off if the company's stock price is driven down by the controversy.
The shares plunged more than 14 percent in January after Markey's initial letters and another 7 percent last week after the company announced it was being investigated by the Federal Trade Commission.
Markey has said his staff did not tell him Ackman stood to gain financially from his actions and claimed he was motivated by concerns that Herbalife's business practices were hurting consumers.
Herr, a selectman in the Boston suburb of Hopkinton, said he mailed his complaint to Sen. Barbara Boxer, a California Democrat who chairs the ethics committee, and Georgia Sen. Johnny Isakson, the Republican vice-chairman.
"All you have to do is connect the dots to easily see that Sen. Markey used his public office to manipulate the stock of a publicly traded company in order to financially benefit a campaign donor," Herr said.
In a statement, Markey did not directly comment on Herr's allegations, but said he has fought for consumers his entire career. Before winning Kerry's seat in a special election, he served as the U.S. representative for Massachusetts' 5th Congressional District from 1976 to 2013.
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