According to his brother, John Kerry plans to take a leadership role in the Senate and he has not ruled out another run for the White House.
From Cameron Kerry: “That’s conceivable … I don’t know why that should necessarily be it. I think it’s too early to assess. But I think that he is going to continue to fight on for the values, ideals, and issues this campaign is about.”
He’s still using the present tense to describe the campaign, so I’d take this will a grain of salt.