A last-ditch attempt by US President Donald Trump and Senate Republicans to dismantle Obamacare collapsed as a third GOP senator has announced her opposition and left the proposal short of the votes needed to pass.

Senator Susan Collins on Monday joined two of her colleagues John McCain and Rand Paul in rejecting the latest bill to end the Affordable Care Act — popularly known as Obamacare. The move came as a major setback to Trump who spent the past week trying to rally support for an attempt to fulfil a seven-year-old Republican promise, the Washington Post reported.

The President had made undoing Democratic former President Barack Obama’s signature healthcare law a top priority since the 2016 campaign.

The bill, sponsored by Republican Senators Bill Cassidy and Lindsay Graham, aimed to eliminate key portions of Obamacare, including the system whereby citizens are fined if they do not obtain health insurance and government subsidies are provided to insurers to expand Medicaid.

“Everybody knows that’s going to fail,” said Senate Finance Committee Chairman Orrin G. Hatch, who led a five-hour hearing on the bill on Monday afternoon. “You don’t have one Democrat vote for it… So it’s going to fail.”

Senator Collins, one of three Republican senators who opposed the last repeal attempt in July, described the latest plan as “deeply flawed”. She expressed concern about cuts to Medicaid as well as the rolling back of protections for people with pre-existing medical conditions.

“Sweeping reforms to our healthcare system and to Medicaid can’t be done well in a compressed time frame, especially when the actual bill is a moving target,” Collins said.

McCain, who killed the last repeal effort with a dramatic middle-of-the-night vote, faulted Republicans for trying to pass sweeping healthcare legislation without the participation of Democrats or extensive public deliberations.

Paul had previously said he would oppose the bill because it did not go far enough in repealing the health law. On Monday, he continued to denounce it as a “fake repeal”.

Republicans hold a 52-to-48 advantage in the Senate. They can lose only two votes from their party and still pass legislation with the help of a tie-breaking vote from Vice President Mike Pence, the Post reported.

Senator Ted Cruz already withheld his support and requested changes to the bill.

Before Collins’s announcement, Trump expressed frustration that Republicans had talked for years about repealing and replacing the Affordable Care Act but failed to deliver now that a Republican was in the White House.

On the “Rick & Bubba Show”, a radio programme, Trump singled out McCain, calling his vote in July “a tremendous slap in the face of the Republican Party”.