He becomes the fifth GOP senator to say he will not back the bill as it is currently written, and the first in Senate Republicans' so-called moderate wing. Some of those lawmakers could eventually change their stances after potential amendments to the plan.

"In this form, I will not support it," Heller said at a news conference with Republican Nevada Gov. Brian Sandoval.

Sen. Dean Heller, R-Nev., said Friday he will not vote for the Senate's Obamacare replacement bill as currently written.

If three Republicans defect, the party cannot reach the majority vote it needs to pass the measure.

Senate Republicans on Thursday morning released a draft of their secretive Obamacare replacement bill, called the "Better Care Reconciliation Act of 2017." The plan would repeal Obamacare taxes, restructure subsidies to insurance customers that are based on their incomes and phase out Medicaid's expansion program. It contains some key differences from the version the House passed last month.

Heller and Sandoval said they believe that Obamacare has problems, but raised concerns with the Senate plan's Medicaid provisions. Heller said he thought the bill would harm the elderly, the disabled and people battling substance abuse, adding that he is "not confident" it will lower costs for consumers.

Sandoval said Medicaid expansion provided coverage to about 210,000 people in Nevada. He added that Nevada "is one of the most improved states in the country" in expanding coverage.

"You have to protect Medicaid expansion states. That's what I want. Make sure we're taken care of here in the state of Nevada," Heller said.

The federal government's share of funding for Medicaid, which is jointly run with individual states, would fall over the course of seven years to end up at around 57 percent of the cost of that program, which offers health coverage to the poor.

Under Obamacare, the federal government had guaranteed that its funding for adults newly eligible for Medicaid because of the Affordable Care Act would fall to no lower than 90 percent of their costs. That expansion program would begin being phased out in 2021, and fully repealed three years later.

Heller's position potentially adds to the difficulties the GOP faces in changing the plan to secure votes from at least 50 of its 52 members. The other four Republicans who said they would not back the current bill — Rand Paul of Kentucky, Ted Cruz of Texas, Mike Lee of Utah and Ron Johnson of Wisconsin — did so because it did not go far enough to repeal Obamacare.

"There are provisions in this draft that represent an improvement to our current health care system, but it does not appear this draft as written will accomplish the most important promise that we made to Americans: to repeal Obamacare and lower their health-care costs," the senators said in a statement Thursday.

Both Paul and Cruz told reporters Thursday that they felt they could get to "yes" votes with tweaks to the proposal. Paul said, "My hope is not to defeat the bill."

But tweaks to appease the four conservatives could alienate Heller and other moderates, or vice versa.

House Republicans barely managed to muster enough votes with a series of last-minute amendments before the chamber passed a highly criticized Obamacare replacement plan last month. That chamber, too, faced opposition from both conservative and moderate pockets.