"It's simply not the answer," the Nevada Republican said at a news conference alongside Gov. Brian Sandoval in Las Vegas. "And I'm announcing today that in this form, I simply will not support it."
Almost immediately, the pro-Trump group America First Policies decided to launch what a source with the group says will be a major television, radio and digital ad buy against Heller -- a remarkable attack on a member of Trump's own party whose seat is endangered in 2018.
It was not immediately clear whether the group intends to back a primary challenger to Heller.
Heller, like four other GOP senators who announced their opposition to the Obamacare repeal bill that was unveiled Thursday, left the door open to negotiations. But he said "it's going to be very difficult to get me to a 'yes'" on the bill.
GOP Senate leaders can only lose two members of their 52-senator caucus in support of the bill in order for it to pass. Heller said he spoke with Senate Republican leaders and Vice President Mike Pence on Friday morning.
Heller said it is a "lie" that the Senate Republican health care bill would lower premiums.
"There isn't anything in this bill that would lower premiums," he said.
Heller, a Republican who is up for re-election in 2018, is the Democrats' top target in the upcoming cycle. Already, Democratic Rep. Jacky Rosen is preparing to challenge him. He said the changes he is seeking to the bill would go in the opposite direction of those sought by other current "no" votes -- conservative hard-liners including Texas Sen. Ted Cruz and Utah Sen. Mike Lee.
Sandoval, a Republican who decided to opt into expanding Medicaid, said 210,000 received health coverage because of the decision. He said the state's uninsured rate declined from 23% before the Affordable Care Act to 11 or 12% afterward.
Sandoval specifically pointed to people who were making a little more than $16,000 per year. "These are our friends, these are our families, and these are our neighbors. ... They are living healthier and happier lives because of that decision -- I don' think that can be overstated enough."
Sandoval said he would do "everything in my power" to make sure those people can maintain the quality of life they currently have. He thanked Heller for working "hand in hand" with him. Sandoval also asked if it's "really realistic" for people earning $16,000 per year to buy insurance on the exchange if they lose Medicaid eligibility.
He pulled out the Senate Republican bill. "There's still an opportunity to make this bill better," he said.
This story has been updated.