The problem he is facing is, if he doesn’t win this time, with all of the help he is getting, he will never be able to win in that district. And he can’t run for the House in another district after losing there or he will be accused fo shopping around for a place he can win. And it is unlikely he could win a Senate race after losing a House run. So he wanted to wait untiil he had a better chance at winning then running in a Republican dominated district.
Folks, you’ve got to read the older threads. 
We’ve been speculating about Joshua* replacing Rob from the first episode he was on.
I didn’t think this was a bad episode - the Prez had some good lines - and it was obviously meant to start new plot lines for the future. But it did feel kinda empty. Christian Slater wasn’t in it at all, nor was Joanna Gleason**, nor any of the security and military types. If C.J. had any lines I’ve already forgotten them. Lily Tomlin needs to get a real character and quick.
And I’m really surprised no one mentioned what I thought was the biggest plot hole I’ve seen in a while. Josh didn’t know where the Vice President had vacationed? Excuse me? There’s a second in the Vice President’s day during which the senior White House staff don’t know where he is and how to reach him? (And not one of the 40 people involved put out the word that the Prez himself had persuaded them to go for Hoynes?) Inexcusable.
As for Rob, I think Lok has it right, but it should be noted that a Democrat, Linda Sanchez, did win a House seat from Orange County. I believe this seat was new, a result of the increased number of seats California got from the new census, so if this is also true in West Wing-land, there’s a chance for Rob.
*needed to distinguish Joshua Malina the actor from Josh Lymon the character.
** who the IMDB hasn’t credited with any West Wing appeances since 1999! What gives?
Oops. Read it wrong. Never mind.