Research 2000 for Daily Kos (1/5-7, likely voters):
Barbara Boxer (D-inc): 49
Arnold Schwarzenegger (R): 40
(MoE: ±4%)
First, here’s the bad news: Barbara Boxer is under the 50% mark that represents relative safety for an incumbent. The rest of the story is pretty good: that’s against Arnold Schwarzenegger, who is the best-known and probably most popular Republican in California, a guy who many people have feared would convincingly defeat Boxer or at least turn this expensive blue-state seat into a top-tier tossup.
And that presumes that Schwarzenegger even runs, which may not be likely at this point, as his popularity (which enjoyed a resurgence just in time for his re-election in 2006) seems to be waning again. The same sample gives him 42/51 favorable/unfavorable ratings (note that almost everyone in the state has an opinion, meaning he’d have to start changing minds instead of just winning over undecideds).
Californians aren’t that red-hot about Boxer either (she has a lukewarm 48/46 favorable/unfavorable). But if Arnold says ‘hasta la vista’ to electoral politics, with the other potentially strong statewide California Republicans (Insurance Commissioner Steve Poizner and ex-eBay CEO Meg Whitman) eyeing the governor’s seat, and with probably none of the GOP House members willing to give up their safe seats on a fool’s errand, it’s possible she might only draw second- (or third-) tier opposition again.