Senate Republican leaders unveiled what they called a "discussion draft" of their long-awaited health care bill, a part of the party's ongoing efforts to repeal and replace the Affordable Care Act.
Five Republican senators have already come out in opposition to the Senate bill, the Better Care Reconciliation Act, jeopardizing its passage.
Critics on both sides of the aisle said the bill, which was drafted behind closed doors by a small group of Senate leaders and committee staffers, has been shrouded in secrecy.
Trump told reporters Thursday that there will be "a little negotiation, but it's going to be very good."
Republicans 'not ready' to support the bill
GOP Sens. Ted Cruz of Texas, Ron Johnson of Wisconsin, Mike Lee of Utah and Rand Paul of Kentucky released a joint statement saying, "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."
They added, "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."
In a separate statement, Paul said he'll oppose the bill "in its current form, but I remain open to negotiations."
"The current bill does not repeal Obamacare. It does not keep our promises to the American people," he said.
On Friday, Sen. Dean Heller, R-Nev., joined the group, saying at a press conference that "this bill is not the answer, it's simply not the answer."
"In this form, I will not support it," said Heller.
Sen. Susan Collins, R-Maine, told reporters Thursday she has "not yet had the opportunity to read the text of the bill, and the details really matter."
"I see some positive features of this bill that are improvements over the House, and I see some negative features based on my first analysis," she said. "I don't like the provision that eliminates federal funding for Planned Parenthood. It makes no sense to single out Planned Parenthood from all the Medicaid providers. There's already a ban against using federal funds for abortions, so there's absolutely no need for that."
A vote from Collins, who has been willing to break from her party, would be key to ensuring the bill's passage.
Senate Republicans can afford to lose only two of their members to pass the bill, assuming Democrats remain united in their opposition.
Republicans acknowledge tough road ahead for bill
As members left a meeting about the bill, many said they were encouraged by their first impressions of the text but were hesitant to say if it would clear the 50 vote threshold for passage.
"There's a lot to digest. It's very complicated," Sen. Pat Toomey, R-Pa., said as he left the gathering.
Some Republicans said they liked how the Senate bill calculates the value of tax credits to help individuals pay for insurance.
While the House bill linked the tax credits to age only, the Senate bill considers age, income and geographical area.
"A person making about $12,000 a year will have more access and a lower cost of health insurance. And that's a really good thing," said Sen. Tim Scott, R-S.C.
But Sen. John Thune, R-S.D., a member of Senate leadership, acknowledged that the draft would not pass in its current form.
"Right now the challenge is, how do we get to 50?" he said.
Senate Majority Leader Mitch McConnell indicated that he wants to get a vote before the July 4 legislative recess.
Democrats and ACA supporters unhappy
The bill's release was met with significant opposition from Democrats and other supporters of the Affordable Care Act.
Under current law, all insurance plans have to include, at a minimum, specified essential health benefits, including ambulance service, hospitalization, maternity care and prescription drug coverage. Under the Senate bill, states would be allowed to apply for waivers from those regulations and essentially scrap them to write their own rules.
As the bill was being unveiled, a large demonstration formed outside McConnell's office, with people in wheelchairs staging a die-in and protesters chanting that no changes be made to Medicaid. Demonstrators were physically removed by Capitol Police officers.
Congressional Democrats were also forceful in their condemnation.
Senate Minority Leader Chuck Schumer slammed the Senate bill as "every bit as bad" as the American Health Care Act passed in the House.
"The president said the Senate bill needed heart. The way this bill cuts health care is heartless," Schumer said Thursday. "The president said the house bill was mean. The Senate bill may be meaner."
He continued, "The Senate Republican health care bill is a wolf in sheep's clothing, only this wolf has even sharper teeth than the House bill."
During her weekly press conference, House Minority Leader Nancy Pelosi said it's important to stop the legislation, which she called "a tax bill disguised as a health care bill."
Top medical organizations call on the Senate to reject the bill
So far, the Senate health care bill has not gotten any backing from top health or medical organizations. The American Public Health Association, the American Psychiatric Association and the American College of Obstetrics and Gynecology released statements urging the Senate to reject the Better Care Reconciliation Act and expressed concerns over the closed-door negotiation process.
"The Senate proposal represents a significant move in the wrong direction, resulting in fewer people having access to insurance, fewer patient protections and less coverage for essential behavioral health care," American Psychiatric Association's CEO and medical director, Saul Levin, said in a statement.
The American Public Health Association attacked the bill's closed-door shaping as "legislative malpractice."
The American College of Obstetrics and Gynecology said, "Despite numerous efforts to collaborate and provide input throughout this process, women's health expertise was rejected. It is reckless for legislation that will have such an immense impact on Americans' lives and the economy to proceed without opportunity for public hearings or any external commentary."
ABC News' John Parkinson, Mariam Khan and MaryAlice Parks contributed to this story.