US Senator John McCain has declared his opposition to the last-ditch effort to repeal and replace "Obamacare", dealing a likely death blow to the legislation and, perhaps, to the Republican Party's years of vows to kill the programme.

Donald Trump and Republican leaders have scrapped their bill to repeal "Obamacare" in a humiliating failure for the US president, when it became clear his flagship manifesto pledge would nosedive in the House of Representatives.