William I. Koch, the Osterville summer resident and fossil fuel magnate who helped bankroll the opposition to the Cape Wind energy project, has emerged as a key backer of Mitt Romney’s presidential campaign, donating $2 million to Restore Our Future, the super PAC supporting Romney’s drive for the White House.

The donations rank Koch’s privately held Oxbow Corp. third among companies that have donated the most to Restore Our Future, behind Perry Homes, the Texas home-building company, and Bain Capital, the Boston private equity firm that Romney cofounded in 1984.

Brad Goldstein, Koch’s spokesman, said Koch is supporting Romney because of a personal relationship that dates to the early 1980s, when Romney was running Bain Capital and Koch was living full time in Massachusetts, and also because of Koch’s disagreements with President Obama’s tax and energy policies.