Four Republican senators said Monday they do not support an early Obamacare replacement draft authored by House lawmakers because it does not go far enough to protect those enrolled in Medicaid.

Sens. Rob Portman of Ohio, Cory Gardner of Colorado, Lisa Murkowski of Alaska and Shelley Moore Capito of West Virginia, said the plan "lacks key protections" for those on Medicaid, which was expanded under Obamacare.

The four lawmakers disclosed their objection to the plan in a letter to Senate Majority Leader Mitch McConnell, R-Ky.

The senators cited a Feb. 10 draft of the healthcare proposal, which Republicans hope to use as a partial replacement to the Affordable Care Act.

House Republican lawmakers are preparing to unveil legislation that would replace the Medicaid expansion with block grants to the states, though it is not clear at what level the federal government would continue to reimburse states for the expansion and to cover states that did not opt for the expansion as part of Obamacare.

"We believe Medicaid needs to be reformed, but reform should not come at the cost of disruption in access to healthcare for our country's most vulnerable and sickest individuals," the four lawmakers wrote to McConnell. "Any changes made to how Medicaid is financed through the state and federal governments should be coupled with significant new flexibility so they can efficiently and effectively manage their Medicaid programs to best meet their own needs."

They advocated for a gradual transition to a new Medicaid program, which the House GOP was considering.

The letter comes after three other Republican senators said they would not back the House draft bill that has been circulating in recent weeks.

A GOP aide said House Republican leaders, with input from the White House, made some changes to the plan over the weekend and it will be released early this week.

Senate Republican support is critical. Republicans hold a 52-seat majority in the Senate and can afford to lose only two GOP lawmakers if they are to pass a measure that repeals Obamacare and includes some elements to replace it.