Chief lieutenants in the Koch brothers' political network lashed out at the Senate Republican health care bill on Saturday as not conservative enough, becoming a powerful outside critic as GOP leaders try to rally support for their plan among rank-and-file Republicans. Tim Phillips, who leads Americans For Prosperity, the Koch network's political arm, called the Senate's plans for Medicaid 'a slight nip and tuck' of President Barack Obama's health care law, a modest change he described as 'immoral.' 'This Senate bill needs to get better,' Phillips said. 'It has to get better.' Some Republican senators have raised concern about cuts to Medicaid, which provides health care coverage to millions of poor and middle-income Americans. Several more conservative senators have voiced opposition because they feel it does not go far enough in dismantling what they call 'Obamacare.' The comments came on the first day of a three-day private donor retreat at a luxury resort in the Rocky Mountains. Invitations were extended only to donors who promise to give at least $100,000 each year to the various groups backed by the Koch brothers' Freedom Partners — a network of education, policy and political entities that aim to promote small government. 'When I look at where we are at the size and effectiveness of this network, I'm blown away,' billionaire industrialist Charles Koch told hundreds of donors during an outdoor evening reception. His brother, David Koch, looked on from the crowd along with Sens. Mike Lee of Utah and Jeff Flake of Arizona. 'We've got to keep doing it at an accelerated pace,' Charles Koch said. No outside group has been move aggressive over the yearslong push to repeal Obama's health care law than the Kochs', who vowed on Saturday to spend another 10 years fighting to change the health care system if necessary. The Koch network has often displayed a willingness to take on Republicans — including President Donald Trump — when their policies aren't deemed conservative enough. Network spokesman James Davis said the organization would continue to push for changes to the Senate health care bill over the coming week. 'At the end of the day, this bill is not going to fix health care,' Davis declared. The network's wishes are backed by a massive political budget that will be used to take on Republican lawmakers, if necessary, Phillips said. He described the organization's budget for policy and politics heading into the 2018 midterm elections as between $300 million and $400 million. 'We believe we're headed to the high end of that range,' he said. On Friday, Nevada Republican Dean Heller became the fifth GOP senator to declare his opposition to the Senate health care proposal. Echoing the other four, Heller said he opposes the measure 'in this form' but does not rule out backing a version that is changed to his liking. Heller, facing a competitive re-election battle next year, said he was opposing the legislation because of the cuts it would make in Medicaid. Senate Majority Leader Mitch McConnell, R-Ky., has said he's willing to alter the measure to attract support, and promised plenty of back-room bargaining as he tries pushing a final package through his chamber next week. Republican leaders have scant margin for error. Facing unanimous Democratic opposition, McConnell can afford to lose just two of the 52 GOP senators and still prevail. At least two of the current opponents, Lee and Texas Sen. Ted Cruz, were among 18 elected officials scheduled to attend the Koch retreat. Two more undecideds were also on the guest list: Flake and Nebraska Sen. Ben Sasse. President Donald Trump continued to push for replacing Obama's health care law, tweeting Saturday: 'I cannot imagine that these very fine Republican Senators would allow the American people to suffer a broken ObamaCare any longer!' The Senate measure resembles legislation the House approved last month that the nonpartisan Congressional Budget Office said would mean 23 million additional uninsured people within a decade and that recent polling shows is viewed favorably by only around 1 in 4 Americans. Charles Koch and his chief lieutenants met privately with Vice President Mike Pence for nearly an hour Friday. Pence, a longtime Koch ally, was in Colorado Springs to address a gathering of religious conservatives. Phillips said it was 'a cordial discussion' about policy, but that neither side asked the other for anything specific. Also Saturday, retired football star Deion Sanders announced plans to partner with the Kochs to help fight poverty in Dallas. The unlikely partnership aims to raise $21 million over the next three years to fund anti-poverty programs in the city where Sanders once played football. The outspoken athlete also defended Koch, who is often demonized by Democrats, as someone simply 'trying to make the world a better place.' 'I'm happy where I am and who I'm with because we share a lot of the same values and goals,' Sanders said when asked if he'd be willing to partner with organizations on the left.