Should Susan E. Rice, the United Nations ambassador, be nominated for Secretary of State, one issue likely to arise during confirmation hearings, aside from the lethal attack on the American Mission in Benghazi, Libya, is her large stock holdings in TransCanada, the company seeking an American permit to build the proposed the Keystone XL pipeline.

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According to financial disclosure documents filed in May, Ms. Rice and her family own from $300,000 to $600,000 worth of stock in the Canadian pipeline company .

Because the State Department oversees transboundary pipelines, the Secretary of State is ultimately responsible for determining whether Keystone XL should go forward.

But her stock holdings in TransCanada, first reported by the Sunlight Foundation and Onearth.org, an environmental publication affiliated with the Natural Resources Defense Council, would surely complicate the issue.

According to the United States Office of Government Ethics, federal law requires executive branch employees to be recused from matters “if it would have a direct and predictable effect on the employee’s own financial interests or on certain financial interests that are treated as the employee’s own.”

The ethics agency also advises that employees may be directed to divest financial investments if they pose a substantial conflict.

Last year, Ms. Rice and her husband earned between $7,500 and $25,000 on their investments in TransCanada, according to financial disclosure forms (which require officials to provide only a range of investments and earnings).

“We need a full team at the State Department, including the secretary, to be free of any conflicts of interest when it comes to tar sands and TransCanada,” said Susan Casey-Lefkowitz of the Natural Resources Defense Council, which opposes Keystone XL.

“I think she would need to sell her stock, and we would fully expect her to do that.”

In an email from Erin Pelton, a spokesperson for Ms. Rice, From Erin Pelton, Spokesperson:

“Ambassador Rice has complied with annual financial disclosure and applicable ethics requirements related to her service in the U.S. government and is committed to continuing to meet these obligations,” read an email sent by Erin Pelton, a spokesperson for Ms. Rice.

Ms. Rice has not been nominated by President Obama to head the State Department, but she is said to be among his top choices.

“Ambassador Rice has complied with annual financial disclosure and applicable ethics requirements related to her service in the U.S. government and is committed to continuing to meet these obligations,” read an email sent by Erin Pelton, a spokesperson for Ms. Rice, on Friday. TransCanada said it was not in a position to speak about potential nominees.

Mr. Obama recently leapt to Ms. Rice’s defense as Congressional Republicans ramped up criticism of her handling of the attack on American diplomats in Benghazi, Libya.

“That is a matter for the president of the United States,” said Grady Semmens, a spokesman for the company.

Ms. Rice, whose husband is Canadian, also has stock in other Canadian energy companies whose pipeline projects would fall under the jurisdiction of the State Department.

Federal filings show that Ms. Rice and her husband own at least $1.5 million worth of holdings in Enbridge, which transports Canadian oil sands crude through the United States. The company was heavily criticized by environmental groups after a 2010 pipeline spill dumped more than 840,000 gallons of oil near Marshall, Mich., leading to the closure of a 39-mile stretch of the Kalamazoo River.

The fate of the Keystone XL, which would ship oil sands crude from Alberta to the Gulf Coast, is expected to be one of the most controversial decisions for the new Secretary of State.

This post has been updated to include response from Ms. Rice’s office.

