VAIL — Republican politicians were gathering near Vail on Sunday for a semiannual retreat aimed at promoting business-friendly policy.

The meetings are organized by two of the nation’s most powerful conservative political donors, Charles and David Koch of the privately held energy giant Koch Industries. Their brother, William Koch, who on Saturday paid $2 million at a Denver auction for a tintype image of Billy the Kid, also is a conservative political donor, but he does not participate in his brothers’ activities.

The guest list is kept secret, and organizers won’t say where the four-day retreat is being held beyond describing it as in the Vail area. Virginia Gov. Bob McDonnell has said he will attend this year’s retreat. Previous guests have included South Carolina Sen. Jim DeMint, conservative commentators Rush Limbaugh and Glenn Beck, and Denver billionaire Phil Anschutz.

On Friday, a spokeswoman for Koch Industries said, “The purpose of this conference is to develop support for the kind of free-market policies and initiatives that can get our country back on the path to economic prosperity and sustained job creation.”

Several dozen left-leaning protesters held an opposition rally in nearby Avon. Common Cause Colorado and others argue that business interests have too much influence over politicians.

Organizers say they are demonstrating against what they call undue influence on politics by deep-pocketed donors such as the Koch brothers, who are prominent funders of Tea Party efforts.

“We’re protesting this mass infusion of millions of dollars into our political system without it being transparent,” said Kjersten Forseth, executive director of ProgressNow Colorado.