SHOKAN, N.Y. — This is the story of how one young couple came to the Hudson Valley with a fortune and big political dreams.

Two years ago, Sean Eldridge and his husband, the Facebook co-founder Chris Hughes, bought a $5 million estate in Garrison, about 50 miles north of New York City. It offered 80 acres of rolling fields and a farmhouse once owned by a Vanderbilt. It would also allow Mr. Eldridge, 26, to run for the local Congressional seat if he chose to.

But that seat appeared unattainable, and soon the couple’s gaze shifted north, to the neighboring district. In January, they bought a $2 million modern home here overlooking a reservoir, laying the groundwork for Mr. Eldridge’s campaign for their new local Congressional seat, New York’s 19th.

Word of Mr. Eldridge’s political plans has delighted the friends who make up his social circle: Donors to his exploratory committee include George Soros, the billionaire financier, and Sean Parker, the tech entrepreneur behind Napster and Spotify.