The Senate allows Bernie much more influence on the national stage than he would have as Governor of a very small state. That said, Bernie still has a chance at the Democratic nomination. There are still 4 debates of the primary season, and Iowa and New Hampshire haven't voted yet. If Bernie wins NH, he will have momentum going into IA. If he wins both, his chances at winning the nomination increase.
Iowa votes first and he's well behind there. He'd need to win both to even have a chance because he's gonna get curbstomped in South Carolina and Nevada.