Washington, Sep 23 (IANS): US Senator John McCain has said he cannot support his fellow Republicans' latest effort to repeal Obamacare, dealing it a potentially fatal blow.

The Arizona senator, who is battling brain cancer, said he "cannot in good conscience" vote for the new plan, which President Donald Trump backs, BBC reported on Saturday.

McCain said it was wrong to pass such far-reaching legislation without input from the Democrats.

It is the second time he has thwarted his party leadership on the issue.

President Trump, speaking in Alabama late on Friday, condemned Republicans who did not back the new legislation, saying: "They don't have the guts to vote for it."

Republicans need 50 votes in a 100-seat chamber they control 52-48 to succeed.

With Democrats united in opposition, there is little margin for error.

McCain's objection could doom conservatives' seven-year campaign to erase Democratic former President Barack Obama's signature legislative achievement.

In a statement on Friday, the Arizona senator said such a bill demands extensive hearings, debate and amendment.

"That is the only way we might achieve bipartisan consensus on lasting reform," he wrote, "without which a policy that affects one-fifth of our economy and every single American family will be subject to reversal with every change of administration and congressional majority."

McCain said he could not support a rushed bill "without knowing how much it will cost, how it will effect [sic] insurance premiums, and how many people will be helped or hurt by it".

One other Republican senator, Rand Paul, is also against the party's latest bill.

Susan Collins indicated on Friday afternoon she may join the opposition.

The Maine senator said she was worried the legislation did not do enough to protect patients with pre-existing conditions.

"I'm leaning against the bill," Collins said at a constituency event.

At least four other Republican senators are undecided: Lisa Murkowski, Dan Sullivan, Rob Portman and Jerry Moran.