Charles Koch stands for a portrait in 2015. (Patrick T. Fallon/For The Washington Post)

COLORADO SPRINGS — The Koch network is working with conservative allies behind the scenes to make changes to the Senate health-care bill that was unveiled this week, declining to endorse the measure as it stands.

As hundreds of donors gathered Saturday for a three-day seminar organized by billionaire industrialist Charles Koch, leaders from the constellation of groups that support his agenda outlined concerns about the draft bill.

“In all candor, we’ve been disappointed that movement is not more dramatic toward a full repeal or rollback (of the Affordable Care Act). But we’re not walking away,” said Tim Phillips, the president of Americans for Prosperity. “We still think this can get done, but the Senate bill needs to get better.”

AFP has spent years mobilizing against Obamacare and battling against Medicaid expansion at the state level. The group came out against the first version of the House health-care bill this winter and helped push for some of the amendments that were added before it ultimately passed last month.

Phillips said the subject came up on Friday night during a 45-minute meeting here between Koch and Vice President Pence. Phillips said no specific “ask” was made of the vice president during the sit-down, which also covered tax reform. “We’ve had productive discussions with them to try getting to the right place,” Phillips said.

Several key players in the ongoing Senate negotiations are at The Broadmoor resort for the network’s twice-annual gathering, including two of the five GOP holdouts: Ted Cruz (Tex.) and Mike Lee (Utah). The No. 2 in GOP leadership, John Cornyn (Tex.), is also here, along with Cory Gardner (Colo.), Jeff Flake (Ariz.) and Ben Sasse (Neb.).

Some conservatives who could determine whether anything that passes the Senate would subsequently clear the House are coming to the Koch conference, including Freedom Caucus Chairman Mark Meadows (N.C.), Jim Jordan (Ohio), Dave Brat (Va.) and Justin Amash (Mich.).

Network officials said they still anticipate spending between $300 million to $400 million on political and policy activity in the 2017-2018 election cycle, a target unveiled earlier this year. They said it will probably be closer to the higher end of that range.

James Davis, who runs communications for the Koch network, lamented that the conversation on health-care reform has focused too much on the number of people who have insurance — regardless of premiums or what kind of care those who have it will receive — and not enough on outcomes. But he also stressed that no single bill is going to fix the system, and that it will be a long-term effort with multiple steps — potentially laying the groundwork for the group to support the bill with minimal changes.

“We effectively engaged to make the House bill better, and that’s what we’re going to try to do here,” Davis told a group of reporters. “We’ve had open channels. … We’re looking at how we can make tangible progress. In the House, with the first bill, there was no real engagement before it came out.”

Medicaid remains a major concern of the Koch network, which campaigned successfully against expansion in many red states. In places like Ohio and Nevada, Republican governors expanded the entitlement despite the opposition of conservative activists.

“We’re going to continue to call them out on this and explain why they’re wrong,” Phillips said.