Senator John McCain said he "cannot in good conscience" vote in favor of his fellow Republicans' latest efforts to repeal the Obamacare healthcare programme.
McCain said it would be wrong to pass such significant legislation without inputs from the opposition Democrats.
This is the second time McCain has thwarted a Republican vote on the issue.
Repealing Obamacare was President Donald Trump's signature campaign promise.
Why McCain's vote is crucial to repeal Obamacare
The Republicans need 50 votes to repeal Obamacare in the 100-seat Senate, which they control with a 52-48 majority.
The Democrats are united in opposition, leaving Republicans little margin for in-party dissent.
Besides McCain, Republican Senator Rand Paul is also against the bill while Senator Susan Collins said she may join the opposition.
Four other Republican senators remain undecided.
McCain: Republicans bill requires extensive hearings, debate and amendment
McCain said bipartisan consensus is essential to achieve "lasting reform" for a policy "that affects one-fifth of our economy and every single American family."
Without consensus, the policy could be reversed every time there's a different administration or congressional majority.
He said he can't support a rushed bill without knowing its cost, impact on insurance premiums and how it would help or hurt people.
Trump, Pence trying to gather support for Obamacare repeal
Trump reportedly called lawmakers and state governors to gain support for the bill. He condemned Republicans who didn't support it, saying: "They don't have the guts to vote for it." Vice-President Mike Pence said this would be the Republicans' "last best chance" to repeal Obamacare.
Opposition to Obamacare repeal bill mounts
Several Republican state governors criticized the Obamacare repeal bill process for being rushed and urged their colleagues to go for a slower, bipartisan approach.
Meanwhile, 16 major medical groups have issued a joint statement opposing the legislation.
A Brookings Institution analysis has found that 32 million fewer Americans would have health insurance by 2027 if the Republican bill gets passed.
McCain's move creates uncertainty over vote
Republican Senate Majority Leader Mitch McConnell planned to introduce the Obamacare repeal bill to vote next week.
However, it remains unclear whether this would take place after McCain and other Republican senators expressed opposition.
The Republicans are still reeling from the massive setback suffered from an attempt at repealing Obamacare in the Senate in July.