Timothy Busch, a California-based donor, said he'd vote for Trump but do nothing more on his behalf.
"He doesn't have enough civility and he's not a man of humility," Busch charged, citing specific complaints about Trump's positions on immigration and trade.
Charles and David Koch have hosted such gatherings of donors and politicians for years, but usually in private.
A few of reporters, including one from The Associated Press, were invited to attend some of the forums. As a condition of attending, reporters were not permitted to identify any donors without their permission.
Koch addressed the presidential contest in a general way on Sunday when reporters were present, but went further during a closed-door meeting later in the day.
He outlined two primary criteria for deciding whether to support a candidate, Wright said: whether the candidate would "believes in and will fight for free markets" and whether he or she has a viable chance to win.
Both Clinton and Trump failed the first test, Wright said.
