Unocal Corp.’s board of directors decided late Thursday not to change its recommendation to accept a $16.6 billion bid from Chevron Corp. instead of CNOOC Ltd.’s competing $18.5 billion offer, according to published reports.

El Segundo-based Unocal didn’t support the takeover by the China-controlled state oil company because it didn’t raise its original offer, reported the Financial Times and Bloomberg News.

CNOOC can still increase its bid up until Aug. 10, when Unocal shareholders are scheduled to vote on Chevron’s cash and stock offer.

But mounting political opposition in the U.S. favors Chevron, which has said its bid is superior because it would close quickly, while CNOOC faces government approval over fears related to national security. The Committee on Foreign Investment will review any proposal for CNOOC and Unocal to merge, once the acquisition has shareholder approval.

Senator Byron Dorgan, D-North Dakota, said Friday that he would introduce a bill to prohibit CNOOC from acquiring Unocal. And Rep. Richard Pombo, R-California, said Thursday that he would also seek to block the transaction with legislation.

CNOOC said Friday that it is still in discussions with Unocal.