“I’ve invited Bernie to sit down explore collaboration – everything is on the table,” she said. “If he saw that you can’t have a revolutionary campaign in a counter-revolutionary party, he’d be welcomed to the Green party. He could lead the ticket and build a political movement,” she said.

Stein said she had made her offer directly to Sanders in an email at the end of the primary season, although she had not received a response. Her surprise intervention comes amid speculation that Sanders will finally draw a line under a bruising Democratic contest by endorsing Clinton’s presidential bid next week.

“If he continues to declare his full faith in the Democratic party, it will leave many of his supporters very disappointed,” she said. “That political movement is going to go on – it isn’t going to bury itself in the graveyard alongside Hillary Clinton.”