The Senate has rejected a measure to repeal parts of former President Barack Obama's health law, dealing a serious blow to President Donald Trump and the GOP agenda.

Unable to pass even a so-called "skinny repeal," it was unclear if Senate Republicans could advance any health bill.

The extraordinarily tense vote -- held in the dead of night -- came down to the wire, with Republican Senator John McCain, only recently diagnosed with brain cancer, casting the deciding vote against the legislation.

"This was a disappointment, a disappointment indeed," Senate Majority Leader Mitch McConnell told colleagues after the vote.