OSTERVILLE, Mass. — If the vast wind farm proposed for Nantucket Sound is ever built, William I. Koch will have a spectacular view of it.

Of course, that is the last thing he wants. Mr. Koch, a billionaire industrialist who made his fortune in fossil fuels and whose better-known brothers underwrite conservative political causes, has been fighting the wind farm, called Cape Wind, for more than a decade, donating about $5 million and leading an adversarial group against it. He believes that Cape Wind’s 130 industrial turbines would not only create what he calls “visual pollution” but also increase the cost of electricity for everyone.

Now, as if placing a bet on the outcome of the battle, Mr. Koch, 73, who has owned an exclusive summer compound here for years, has acquired an even grander one — Rachel Mellon’s 26-acre waterfront estate in the gated community of Oyster Harbors, for $19.5 million. He has also bought the nearby 12-plus-acre Dupont estate. All of this adds up to a prime perch over Nantucket Sound.

“I love the area,” Mr. Koch said in an e-mail. “The ability to acquire a special property where I can create a family compound for my children and extended family was and is very meaningful to me.” (His current home, in the same gated community, is on the market for $15 million.)