Updated at 6 p.m. ET June 23

Nevada Sen. Dean Heller on Friday became the latest GOP lawmaker to voice concerns about the Senate health care bill — a development that further complicates Republican efforts to repeal and replace the Affordable Care Act.

"I cannot support a piece of legislation that takes away insurance from tens of millions of Americans and hundreds of thousands of Nevadans," Heller said at a news conference back in Nevada.

It was a stunning indictment of his own party's crucial legislation from Heller, who also said that it was a "lie" that the bill would lower insurance premiums.

Heller is the most vulnerable Republican senator up for re-election in 2018, and he represents a state that accepted the Medicaid expansion under the ACA, more commonly known as Obamacare.

Political pressure from the White House was already mounting against Heller, with the outside group America First Policies, which supports President Trump, was preparing a major ad buy targeting the Nevada senator.

"Obamacare is collapsing. This is a crisis for the American people. There is no excuse for any Republican or Democrat to oppose the Senate health care bill outright," Brian O. Walsh, the group's president, said in a statement. "Senator Heller, who once claimed he would focus on 'repealing and replacing Obamacare with a willing partner in the White House,' appears to be heading down a path with Nancy Pelosi, Chuck Schumer and the radical left. We sure hope he changes his mind as he learns more about the bill. We at America First Policies will make certain that citizens know who stood in the way of repealing and replacing Obamacare, and we will ensure the people's voices are heard."

Heller did indicate he is open to negotiations on the bill that could move him to a yes vote, but any movement toward the center in the bill could risk further alienating a group of conservatives who announced on Thursday that they do not support the draft legislation, saying that the bill didn't go far enough for them. But, like Heller, they indicated they are open to negotiating.

"Currently, for a variety of reasons, we are not ready to vote for this bill, but we are open to negotiation and obtaining more information before it is brought to the floor," Sens. Rand Paul, R-Ky.; Ted Cruz, R-Texas; Mike Lee, R-Utah; and Ron Johnson, R-Wis., said in a statement Thursday afternoon.

With Republicans in the Senate holding only a 52-48 seat edge, they can afford only two defections to get to a 50-50 tie, allowing Vice President Pence to then presumably break any logjam. However, the four senators do appear open to negotiations and amendments that could turn their "no" to a "yes."

"There are provisions in this draft that represent an improvement to our current healthcare 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 healthcare costs," the quartet continued in their statement.

The Tea Party-aligned group FreedomWorks also said in s statement that the bill doesn't live up to promises by Senate Majority Leader Mitch McConnell, R-Ky., of a full repeal of the ACA.

"Unfortunately, the Senate bill is an amendment to ObamaCare, not a repeal of it," said FreedomWorks President Adam Brandon.

Termed the "Better Care Reconciliation Act," the Senate's answer to the House's efforts to repeal and replace the ACA was finally released Thursday morning after weeks of secret negotiations.

As NPR's Danielle Kurtzleben reports, the BCRA is similar in many ways to the House's health care alternative, the American Health Care Act, that passed last month. The bill "rolls back the ACA's Medicaid expansion — making for deep spending cuts to that program, compared with current law. The Senate bill also proposes eliminating many ACA taxes, and the employer penalties associated with the employer and individual mandates would be repealed retroactively, dating to the start of 2016. And as in the House bill, young adults up to the age of 26 could stay on their parents' health care plans."

The bill in its current form may not just alienate conservative voters and lawmakers who think it doesn't go far enough but also may not assuage moderates like Heller and others enough to get their votes, either.

Annie Clark, a spokeswoman for Sen. Susan Collins, R-Maine, said the centrist has some misgivings about the bills as well.

"Sen. Collins will carefully review the text of the Senate health care bill this week and into the weekend. She has a number of concerns and will be particularly interested in examining the forthcoming CBO analysis on the impact on insurance coverage, the effect on insurance premiums, and the changes in the Medicaid program," Clark said. "She has met with and heard the concerns of many Mainers about their health care challenges, and she will continue to do so as she studies the impact of this legislation on Maine and the nation.

Predictably, Democrats were firm in their opposition to the GOP bill. Sen. Elizabeth Warren, D-Mass., didn't mince words when she ripped into the bill on the Senate floor earlier Thursday.

"These cuts are blood money. People will die," Warren said. "Let's be very clear. Senate Republicans are paying for tax cuts for the wealthy with American lives."

Senate Minority Leader Chuck Schumer, D-N.Y., said that the new Republican bill was "every bit as bad as the House bill" and "even worse" in some ways.

"This bill will result in higher costs, less care and millions of Americans will lose their health insurance, particularly through Medicaid," Schumer said. "The way this bill cuts health care is heartless."

Former President Barack Obama, whose signature legislation is the target of GOP repeal efforts, wrote in a lengthy Facebook post that the Republican plan "is not a health care bill."

"It's a massive transfer of wealth from middle-class and poor families to the richest people in America. It hands enormous tax cuts to the rich and to the drug and insurance industries, paid for by cutting health care for everybody else. Those with private insurance will experience higher premiums and higher deductibles, with lower tax credits to help working families cover the costs, even as their plans might no longer cover pregnancy, mental health care, or expensive prescriptions. Discrimination based on pre-existing conditions could become the norm again. Millions of families will lose coverage entirely."

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