Would Richard Riordan have won reelection as Gov. of Cali in 2006?

[Roll Roons]

Yes. Arnold won reelection by quite a lot despite the national wave, and there's no reason to think Riordan would have been less popular than him.

[Roll Roons]

Riordan would have very likely won another term. In general, in 2006 popular GOP governors were not often voted out anyway, and even some unpopular ones survived. Rell, Perry, Lingle, all survived.
The Dem wave was strongest in House and Senate.

Rell was extremely popular. She was the Charlie Baker of the 2000s.

[Roll Roons]

Riordan would have very likely won another term. In general, in 2006 popular GOP governors were not often voted out anyway, and even some unpopular ones survived. Rell, Perry, Lingle, all survived.
The Dem wave was strongest in House and Senate.

Bob Ehrlich

He was the only incumbent who lost. All of the other gubernatorial gains that year came from open seats.

I still don't completely understand why he lost despite being relatively popular. It was not partisanship alone, because Trump was way more unpopular in Maryland than Bush and Hogan won reelection by double digits.