A prominent closed-door conservative conclave is set to begin this weekend in the Vail area, and liberal protesters are planning a rally in response.

The billionaire brothers Charles and David Koch of the privately held energy giant Koch Industries are playing host to a semiannual retreat near the resort town of Vail starting Sunday. The retreats are closed to the public and the press, but news of the meeting leaked out when Virginia's Republican governor, Gov. Bob McDonnell, listed the gathering on his schedule.

The full invitation list for the upcoming retreat hasn't been released. Previous guests at Koch retreats include Rush Limbaugh and South Carolina Sen. Jim DeMint. A retreat in Aspen last summer included conservative commentator Glenn Beck and Denver billionaire Phil Anschutz.

A spokeswoman for Koch Industries declined to tell The Associated Press on Friday who would be attending this year's retreat.

In an emailed statement from spokeswoman Nancy Pfotenhauer 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."

The gathering prompted several left-leaning groups in Denver to plan a protest rally in nearby Avon on the first day of the retreat. Organizers say they hope several dozen people will join to demonstrate 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 through their group Americans for Prosperity.

"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.

A similar protest held near the last Koch retreat near Palm Springs, Calif., in February drew several hundred picketers.

Common Cause, the political watchdog group that organized that rally, later apologized after hateful comments made by some attendees were videotaped by a conservative activist and circulated online.

Common Cause Colorado director Jenny Flanagan promised her group's rally this weekend will be civil.

Colorado AFL-CIO sent members an email Thursday asking them to join to protest.

Another Koch brother with property in Western Colorado, William Koch, is not participating in the retreat. William Koch owns mining and energy company Oxbow Corp. and is a prominent donor to mostly Republican politicians, but does not participate in the political activities of his brothers.

Read more: http://www.sfgate.com/cgi-bin/article.cgi?f=/n/a/2011/06/24/state/n125422D99.DTL#ixzz1QKO6hPqY