Senator John McCain has said he will oppose the Republicans' latest healthcare proposal, likely dooming his party's effort to repeal and replace Obamacare.

In a statement, the 81-year-old said he would not support the Graham-Cassidy bill, written by Republican senators Bill Cassidy and Lindsey Graham, one of his closest friends in the Senate.

"I cannot in good conscience vote for the Graham-Cassidy proposal," Mr McCain said. "I believe we could do better working together, Republicans and Democrats, and have not yet really tried. Nor could I support it without knowing how much it will cost, how it will effect insurance premiums, and how many people will be helped or hurt by it."

"I take no pleasure in announcing my opposition," he added.

Mr McCain, who has brain cancer, had also helped kill the Republicans' previous Obamacare repeal effort in July, saying he did not believe in forcing a healthcare bill through Congress without at least some Democratic support.

Republicans assert that Obamacare, otherwise known as the Affordable Care Act, has destabilised individual markets for health insurance and has forced consumers to buy insurance they do not want or cannot afford.

Following the Republicans' failure to dismantle Barack Obama's signature healthcare law this summer, stronger bipartisan efforts began to "stabilise and strengthen" the individual markets.

However, that bipartisanship on healthcare came to a standstill after Mr Graham and Mr Cassidy introduced their bill last Wednesday, reviving a fight many in Washington thought was over despite Donald Trump's repeated calls for Republicans to fulfill their promise to repeal and replace Obamacare.

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Republicans still have until the end of the month before procedural rules in the Senate make it more difficult for the party to do away with the ACA. But with Mr McCain coming out against the bill, the party's last-ditch attempt to repeal the law may not live to see another day.

While most Senate Republicans would likely vote in favour of the Graham-Cassidy bill if a vote was held, three 'no's' from the party would kill the legislation.

Republican Senator Rand Paul has already said he cannot support the proposal because it doesn't do enough to repeal Obamacare. And Susan Collins has also expressed broad concerns about the bill, suggesting that she would also vote against it.

The Graham-Cassidy proposal has been marketed as a way to give states the ability to run their own healthcare programmes. But the Centre on Budget and Policy Priorities, a left-leaning think tank, estimates that the legislation would push millions off their health insurance and weaken protections for people with pre-existing conditions.

Health insurers have come out against the bill as well as doctors, hospitals, AARP, patient advocates, multiple governors from both political parties and others.