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The hedge fund manager William A. Ackman walked the halls of Congress last year, telling any lawmaker who would listen that he believed Herbalife, a nutritional supplements company, was a fraud. He finally got a break on Thursday, when Senator Edward J. Markey of Massachusetts sent letters to federal regulators urging them to investigate the company.

The action introduced a political element into what has become a billion-dollar brawl among prominent investors. Mr. Ackman contends that Herbalife is a pyramid scheme and wagered that its stock was worthless. The company has forcefully denied Mr. Ackman’s assertions, spending millions of dollars last year to defend its reputation. And it has found champions elsewhere on Wall Street, with several of Mr. Ackman’s rivals buying the company’s shares, taking the opposite side of his bet. That helped drive the stock price skyward, creating hundreds of millions of dollars in paper losses for Mr. Ackman.

Now joining the fight on Mr. Ackman’s side is a freshman senator with no obvious close connection to him, but certainly some awareness.

A spokeswoman for Mr. Markey said that Mr. Ackman had met with Mr. Markey’s staff last fall to discuss Herbalife, but that Mr. Markey had not met Mr. Ackman. Staff members for Mr. Markey have also met with lobbyists who represent Mr. Ackman about Herbalife, said the senator’s spokeswoman, Giselle Barry.

Investors in Herbalife, whose stock dropped more than 10 percent on Thursday after Mr. Markey’s letters were disclosed, noted that many of the points raised in the letters — which were sent to the Securities and Exchange Commission and the Federal Trade Commission — had appeared months earlier in documents Mr. Ackman released to discredit the company.

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While federal campaign filings do not show that Mr. Ackman contributed directly to Mr. Markey’s campaign, they do show that he donated $32,400 to the Democratic Senatorial Campaign Committee on April 30 — the same day Mr. Markey won the Democratic primary to fill the seat vacated by John F. Kerry. A little more than a month after Mr. Ackman made his donation, the Democratic senatorial committee gave $45,400 to Mr. Markey’s campaign, though it is not clear where the money came from.

Both the Democratic and Republican Senate campaign committees are allowed to contribute up to $45,400 directly to candidates, and they routinely give the maximum amount to all of their candidates in a competitive race. Markey was among the recipients for Democrats, and overall the Democratic senatorial campaign committee spent more than $1.8 million related to Markey’s race.

Mr. Markey was not aware that Mr. Ackman had donated money to the Democratic senatorial committee and that his sister — who donated $500 to Mr. Markey’s campaign — had contributed money to him when he sent the letters on Thursday, Ms. Barry said.

In a written statement, a spokeswoman for Mr. Ackman’s firm, Pershing Square Capital Management, said that federal election filings showed that “Mr. Ackman has never given a political donation to Senator Markey.”

“Again, as public filings show,” the spokeswoman said, “Mr. Ackman has a record of donating to Democrats and Republicans, including the Senate campaign committees of both parties, over many years.”

In sending the letters to the S.E.C. and F.T.C., Mr. Markey was following a path that Mr. Ackman blazed last year as he sought to rally regulators and lawmakers around his cause.

Herbalife, a multilevel marketing company based in Los Angeles that sells vitamins and drink mixes through a network of individual distributors, first came under attack from Mr. Ackman in late 2012. He has wagered $1 billion that its stock would fall to zero.

Under financial pressure as the stock rose, Mr. Ackman made the rounds among regulators, meeting with the S.E.C., the F.T.C. and the office of the New York attorney general, in a quest to discredit the company. He released reams of documents, whose points were echoed in Mr. Markey’s letters on Thursday.

“I don’t think it’s entirely a coincidence,” said Rommel T. Dionisio, an analyst at Wedbush Securities, “that you see United States politicians making these inquiries of the F.T.C. to investigate Herbalife just months after these allegations have been put forth by Pershing Square.”

In the letters, Mr. Markey said he had heard complaints about Herbalife from constituents. One family in Norton, Mass., said it lost $130,000, including its entire 401(k), from buying Herbalife sales leads and products as distributors, according to the letters and Ms. Barry.

Another Massachusetts resident said that she was pressed to buy more products to qualify for a higher level in the distribution network and that she was “encouraged to stay in the program even after she said she wanted out,” Mr. Markey wrote.

“I have seen reports from Massachusetts residents that suggest Herbalife is a pyramid scheme,” wrote Mr. Markey, a member of the Senate Committee on Commerce, Science and Transportation.

Mr. Markey also sent a letter to Herbalife, and a spokeswoman said in a statement on Thursday: “We received the letter from Senator Markey this morning and look forward to an opportunity to introduce the company to him and address his concerns at his earliest convenience.” Representatives of the S.E.C. and F.T.C. declined to comment.

In the letter to Herbalife’s chief executive, Michael O. Johnson, Mr. Markey asked several questions about the company’s business, including pointed requests that reflected Mr. Ackman’s concerns.

“How much profit (net earnings after expenses) can the average distributor expect to make from retailing to non-distributors (i.e., people who are not directly involved in Herbalife themselves)?” Mr. Markey asked.

The letter also asked for “the correct number of sales outside the network as a percentage of total sales” for each of the last five years and information on these sales measured by product, quantity and dollars.

Political contributions don’t necessarily come with strings attached, said Martin H. Peretz, the former editor of The New Republic who is a longtime friend of Mr. Ackman’s and was an early investor in his fund.

Mr. Peretz also said that, compared with the vast wealth of Mr. Ackman, who is a billionaire, the contribution to the senatorial campaign committee last year was small.

“$10,000 for people in this stratum of society — hey, that’s a night out on the town,” he said.