I think he is just trying to get his base out. The if-Obama-says-it-then-it-must-be-a-good-thing response. He’s really telling them, “If you want to beat them, y’all have to vote.”
Mandatory voting is an anti-freedom idea that only someone like him would even allow himself to suggest.
His base never left him (as evidenced by the 2012 election); his opposition is dying off and being replaced by apathetic non-voters.