"Today, we find out that there is now a fourth version of the Graham-Cassidy proposal, which is as deeply flawed as the previous iterations," Collins said in a statement. "The fact that a new version of this bill was released the very week we are supposed to vote compounds the problem."
Collins joined Sens. John McCain, R-Ariz., and Rand Paul, R-Ky.
"Obviously this was an issue to which I've given a great deal of thought because there are many problems in the Affordable Care Act that do need to be fixed. However it was clear to me that the Graham-Cassidy bill was not the answer," Collins told ABC News.
Collins said she had three “major concerns” with the proposed legislation from Sens. Lindsey Graham, R-S.C., Bill Cassidy, R-La., Dean Heller, R-Nev., and Ron Johnson, R-Wis. Among them were what she called the "sweeping changes and cuts" in the Medicaid program.
“The CBO’s analysis on the earlier version of the bill, incomplete though it is due to time constraints, confirms that this bill will have a substantially negative impact on the number of people covered by insurance,” Collins quipped.
Off the floor, Collins told reporters President Donald Trump called her in an attempt to sway her vote. She said she told him she's a likely "no" but would take another look at the revised bill. Vice President Mike Pence also called the senator at her home in Maine over the weekend, she said.
"I had a very cordial discussion with the president today. I'm not going to go into the details of the conversation but it was completely cordial, as was my conversation with Vice President Pence and with [Health and Human Services] Secretary [Tom] Price and with many other members of the administration. I appreciated their input, it's just at the end, I can't support this bill," she told ABC News.
Just minutes before Collins announced her vote, the Congressional Budget Office released a preliminary analysis of the latest version of the Graham-Cassidy estimating that “millions” would lose health care.
“The number of people with comprehensive health insurance that covers high-cost medical events would be reduced by millions compared with the baseline projections for each year during the decade, CBO and JCT estimate,” the analysis released Monday states. “That number could vary widely depending on how states implemented the legislation, although the direction of the effect is clear.”
Collins’ "no" vote may well be the final nail in the coffin for the Graham-Cassidy bill. The bill needs 52 votes to pass before the Sept. 30 deadline, and with three Republican senators standing in opposition, the bill is essentially dead on arrival.
ABC News' Trish Turner contributed to this report.