COLORADO SPRING — 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.