Senator Joe Manchin threatens to sink signature Biden bill
"I cannot vote to continue with this piece of legislation," Joe Manchin told Fox News of the $1.9tn (£1.4tn) bill.
Fellow Democrats, including President Joe Biden, have spent weeks trying to secure Mr Manchin's crucial support.
All 50 Democrats are needed to pass the bill in the Senate, with opposition Republicans staunchly opposed.
The Build Back Better Act is a sweeping social spending and climate package, which is considered a key pillar of Mr Biden's agenda.
It would increase funding for government health-insurance programmes, expand tax credits for low-income families, provide government-funded universal preschool, and invest hundreds of billions of dollars in addressing climate change. It would be mostly paid for by raising taxes on the wealthy and corporations.
It passed the House of Representatives last month following a vote that largely fell along party lines, but was expected to face significant hurdles in the Senate.
Republicans say the legislation will fuel inflation and damage the economy.
On Sunday, the Democratic West Virginia senator said he had "worked as diligently as possible... to determine the best path forward despite my serious reservations".
"My Democratic colleagues in Washington are determined to dramatically reshape our society in a way that leaves our country even more vulnerable to the threats we face.
"I cannot take that risk with a staggering debt of more than $29tn and inflation taxes that are real and harmful to every hard-working American at the gasoline pumps, grocery stores and utility bills with no end in sight," Mr Manchin said. Read More…