Sen. Bernie Sanders (I-Vt.) said late Wednesday that he cannot support the Democrats' healthcare reform legislation in its current form.
Earlier on Wednesday, Sanders described himself as "undecided" on the bill.
Sanders, who favors a single-payer healthcare system, said that he has informed the White House and Democratic leadership of his position.
"As of this moment. I am going to do my best to make this bill a better bill, a bill that I can vote for but I’ve indicated both to the White House and the Democratic leadership that my vote is not secure at this point," he said.
Sanders words come as Democratic leaders are now facing heat from the left over several compromises made in order to attract centrist votes.
Senate Majority Leader Harry Reid (D-Nev.) stripped the bill of the public option and the Medicare buy-in provision, two measures popular among liberals. Sanders, who is a self-described "democratic socialist" is the first left-leaning senator to indicate he cannot vote for the new compromise bill.
Other liberals have either said they would vote for the bill or have refrained from stating their position.
President Barack Obama and Democratic leaders have said they would like to pass the bill by Christmas.
Sanders withdrew his 767-page amendment that would have implemented a single-payer healthcare system after Republicans forced the Senate clerk to read it aloud, thereby stalling the healthcare debate.