The latest bid backed by US President Donald Trump to dismantle his predecessor's health care law likely unraveled Monday when a crucial third Republican came out against the plan.

"This is simply not the way that we should be approaching an important and complex issue that must be handled thoughtfully and fairly for all Americans," Senator Susan Collins said in a statement announcing her opposition to a bill which replaces the Affordable Care Act with a system of block grants to states.

Collins joined senators John McCain and Rand Paul as firm no votes on the legislation, which Republican leaders had hoped to put to a vote before a September 30 deadline that would allow them to pass it with a simple majority.

Republicans hold 52 seats in the 100-members Senate and can afford just two defectors.