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Weeks after the spill, a Landrieu spokesman said there were no plans to return the donations in response to an inquiry from the Center for Responsive Politics.

“Campaign contributions, from energy companies or from environmental groups, have absolutely no impact on Sen. Landrieu’s policy agenda or her response to this unprecedented disaster in the Gulf,” the spokesman, Aaron Saunders, told the Center for Responsive Politics.

The oil company’s employees and political action committee historically have given more political donations to Republicans as compared with Democrats, though the gap has narrowed in recent years.

Mr. Levinthal said BP-tied contributions ran by a 3-to-1 ratio in favor of Republicans about a decade ago but that Republicans have received 56 percent compared with 43 percent to Democrats this election cycle.