This is the dramatic moment Senator John McCain defied Donald Trump to vote "no" on his own Republican Party's attempt to reform the Affordable Health Care Act known as Obamacare.

To the visible shock of many colleagues on the Senate floor, the footage shows the veteran Arizona politician pause, before loudly proclaiming "no".

In response, audible gasps can be heard on the floor, with some Democrats clapping as he returned to his chair.

His vote effectively was effectively the decider, killing the so-called "skinny" repeal, which would have scaled back some of the more controversial provisions, is the third failed attempt to repeal Obamacare.

It would have resulted in 16 million people losing their health insurance by 2026, with insurance premiums increasing by 20 per cent, according to the Congressional Budget Office (CBO).

His fellow Republicans Susan Collins and Lisa Murkowski, also voted against the bill, which needed a simple majority to pass.

Ahead of the vote, Mr McCain had not revealed his intentions, but told reporters to "watch the show” as he entered the chamber.