The first official candidate...

... is Andrew Swan. As well, at the press conference where he announced his candidacy, he was endorsed by Oswald and Struthers, so we know they're not running either. Also endorsing him was Nancy Allan, and according to Endless Spin Cycle backbenchers Erin Selby, Sharon Blady, and Bonnie Korzeniowski were also there.

As of writing, nobody else has declared yet.