He's gonna switch parties and get a free ride through the nomination process like Clinton did.
The only question is if the other ambitious Democrats like Gillibrand, Harris, Warren and Booker will accept this.
I think Warren might get a little overly ambitious, because she wants to become Sanders 2.0, and Gillibrand has basically devoted herself to opposing Trump, which means she HAS to run in 2020 or all her theatrics will loose their value.