Usually, when an incumbent can’t get above 50% in a poll, holding the seat begins to look like a long shot.  In California, Rasmussen’s latest poll shows that Boxer can’t get above 47% against any of the three Republicans vying for the nomination to run against her.  Two of them manage to get within the margin of error in the poll:

For the second month in a row, incumbent Democratic Senator Barbara Boxer is in surprisingly tight races with three potential Republican challengers in California.

Most troubling for Boxer in the latest Rasmussen Reports telephone survey of likely voters in the state is her continuing inability to cross the 50% threshold against any of the GOP hopefuls. Incumbents who capture less than 50% of the vote at this stage of the campaign are considered vulnerable. …

Boxer leads former Hewlett-Packard CEO Carly Fiorina 46% to 42%, virtually unchanged from January. Another seven percent (7%) would vote for some other candidate, while five percent (5%) are undecided. Boxer led Fiorina by nine points in November and 10 points in September.

The incumbent also holds a four-point lead over former Congressman Tom Campbell, 45% to 41%. Four percent (4%) like another candidate, and 10% are undecided. Boxer held the same lead over Campbell, just after he joined the race in January.

Andrew Malcolm wonders whether the dump-Democrats tidal wave can reach California:

For a Democrat in a Democrat state that gave Barack Obama 61% of its votes in 2008 (and still likes him more than many other places) to be mired in the mid-40’s is a sign of real trouble. This is especially so given the fact that disgruntled voters gave Democrats control of the House, Senate and White House in 2008, expecting something to happen beyond another[C]ongressional payraise. Voters appear to be looking at a the stubborn ineffectiveness of the much-vaunted economic stimulation bill, continued high unemployment and waning Obama approval. It’s not like they don’t know Boxer after all her years in the state’s public life. To help with the warmth of money, Boxer will have Al Gore headline a fundraiser for her next weekend.

Yes, sure, incumbents retain huge powers and money advantages over challengers and it’s a long time until November, when the five-term ex-representative Boxer will turn 70. But nationally, indications are growing that 2010 could be a tidal wave election beyond the usual midterm swing with voters believing in a different kind of change to believe in.

Is it possible they could dump out a Democrat even in California?

Looking at the internals of the poll, it’s possible — but it really depends on turnout.  Without Obama at the top of the ticket, Democrats can’t count on the kind of flood-the-zone turnout they got in 2008.  There will be a battle to replace Arnold Schwarzenegger as governor, but none of the candidates have the candlepower to provoke a massive turnout on their own.

Throughout the rest of the country, independents have fueled a backlash against Democrats.  That’s not so much the case in California.  Each of the Republicans edges Boxer in unaffiliated voters, but not by much.  Fiorina does best at 45/40, Campbell slightly less well at 42/38, while DeVore gets 41/40.  Those aren’t numbers among likely voters that portend a seat change, at least not at the moment.  However, her favorability among independents is well under water, at 45/51, never a good sign for an incumbent, and more strongly disfavor her than strongly support her, 30/39.

Another factor in Boxer’s favor: Obama is still popular among Californians, more so than nationally.  Rasmussen puts his approval rating at a health 58% in the Golden State, with only 41% disapproving.  He gets the exact same split among independents, which tends to argue that (a) Boxer is really seen as extreme and/or ineffective, and (b) the reaction to the Obama agenda seen in other states won’t be as much of a factor in California.

The real wild card may be instead the state-level anti-incumbent fervor.  The state is mired in financial meltdown, and Democratic control of the state legislature makes them particularly vulnerable to a big anti-Democrat turnout.  Rasmussen didn’t poll on legislative elections, but that could become a factor in November, especially after the budget process this summer.   Without that, though, don’t get your hopes up for a Republican takeover quite yet.