U.S. companies scored a long-sought win Wednesday when theSecurities and Exchange Commission (SEC) approved new rules thatare expected to make it harder for activist investors to push forchanges in corporate strategy.

The main target of the SEC's overhaul is firms such asInstitutional Shareholder Services (ISS) and Glass Lewis & Co.,which are paid by pension funds and other institutional investorsto advise shareholders on how they should vote their stock. Thecompanies—known as proxy advisory firms—have significant influencein whether activist campaigns succeed because investors oftenfollow their recommendations in board elections.

The U.S. Chamber of Commerce and other business groups havelobbied the SEC for years to rein in proxy advisers, arguing thatthe firms are conflicted and should be more aggressively regulated.The SEC took the first step in toughening oversight last November,when it proposed a number of rule changes.