In a letter to Senate Majority Leader Mitch McConnellon Monday, Republican Sens.Rob Portman, Shelley Moore Capito, Cory Gardner, and Lisa Murkowski laid out concerns regarding the repeal and replacement of the Affordable Care Act, in what could be the beginning of serious trouble for the GOP's plan to overhaul the healthcare law.The four senators wrote that they were specifically concerned with the leaked draft version of the House GOP's Obamacare repeal and replacement bill as it pertains the expansion of the federal Medicaid program.Under Obamacare, states were allowed to expand their Medicaid program to include Americans that make up to 137% of the federal poverty line. So far, 34 states and the District of Columbia took funding fromthe federal government to expand the program, leading to coverage for over 11 million people who otherwise would not have had insurance.The senators wrote they wereconcerned that the changes to funding in the leaked House legislationwould leave states stretched to keep suchpeople covered. All four senators come from a state that has expanded Medicaid."While we support efforts to repeal and replace the Affordable Care Act and make structural reforms to the Medicaid program, we are concerned that the February 10th draft proposal from the House of Representatives does not provide stability and certainty for individuals and families in Medicaid expansion programs or the necessary flexibility for states," the letter to McConnell said.The House GOP bill would shift the current funding structure and would put more of the funding onus on the states, which may be difficult to sustain given already stretched state budgets.Inthe letter, the senators advocated for more "flexibility" for states that want to expand Medicaid and urged a longer timeframe for the transition of Medicaid programs."As the largest payer of mental health and substance use services in the United States, it is critical that any healthcare replacement provide states with a stable transition period and the opportunity to gradually phase-in their populations to any new Medicaid financing structure,"the senators wrote.The senators suggested they would not support the draft plan in its current form. It would represent a significant blow to GOP efforts to repeal and replace the law, as the loss of four votes would almost certainly kill Republican efforts to pass legislation."However, the February 10th draft proposal from the House does not meet the test of stability for individuals currently enrolled in the program and we will not support a plan that does not include stability for Medicaid expansion populations or flexibility for states," the letter said.In another sign of worry for GOP leadership, three more conservative-leaning GOP senators ' Rand Paul, Ted Cruz, and Mike Lee ' have pledged to vote against the House GOP bill because it does not go far enough in repealing the ACA. If those three senators vote against the bill, it would also out the GOP under the 50-vote threshold absent Democratic defections.SEE ALSO:Paul Ryan's plan to repeal and replace Obamacare is expected to come out soon ' here are the biggest fights he'll faceJoin the conversation about this storyNOW WATCH: Obama's White House photographer has been trolling Trump on Instagram Click here to read full news..