At stake was Coburn's amendment to allow mentally ill military veterans to buy guns by stripping away part of a landmark gun-control law.
Coburn's amendment was expected to pass if he managed to attach it to a series of votes Friday needing just 51 ayes. He threatened to block the defense bill if any senator objected.
But Schumer said, "I object."
Later Schumer explained, "It was a showdown. I felt morally compelled to do it."
It bars gun purchases or possession by anyone officially deemed to be a "mental defective" -- meaning they are a danger to themselves or others or incapable of managing their affairs.
Coburn argued that the law strips veterans of their Second Amendment rights. Mentally ill veterans should be able to buy or have guns -- unless a court finds them to be a danger.
"All we're asking for the veterans of this country is that if their rights are taken away, that it be adjudicated by a judge or a magistrate . . . rather than a social worker at the V.A.," Coburn said Thursday night.
Schumer responded, "If you're a veteran or not, and you have been judged to be mentally infirm, you should not have a gun," noting that a mentally ill veteran wouldn't wind up in court until after being arrested.
He said he objects to measures rarely, while Coburn "has made himself a legislative powerhouse" by doing it regularly.
For a moment it was tense.
But Coburn backed down, for now. "I will say today, I will not object . . .," he said.
Coburn did not respond to calls and emails for comment.