President Obama was bombarded by nearly 300 groups representing business and manufacturers on Wednesday, urging him to scrap new rules for reducing smog-forming ozone as the costliest regulations in history.

"The need for balanced government policies and reasonable flexibilities has never been greater, and no single regulation threatens to disrupt this balance more than EPA's ozone rule," the groups told the president in a letter sent to the White House.

The National Association of Manufacturers, the Business Roundtable and the U.S. Chamber of Commerce — the largest business lobbies — sent the letter to Obama, signed by 260 other trade organizations, one day after kicking off a major ad campaign to raise awareness of the harmful effects of the regulation.

The manufacturers started the multi-million dollar ad campaign on Tuesday, targeting lawmakers before they adjourn for the August congressional recess. The campaign is part of an eleventh-hour lobbying blitz against the proposed rules before they go into effect Oct. 1.

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