The latest Field Poll is out, and Senator Barbara Boxer has cause for great concern.

The word in Washington is already that Boxer, a 17-year incumbent, is among the most vulnerable Democrat Senators seeking re-election next year. Despite being a Democrat incumbent in a blue state that that voted strongly for Obama, Boxer consistently polls well below her colleague Sen. Dianne Feinstein (D) and evokes much stronger negative responses from voters in polls than her fellow California senator.

After last week’s Field Poll, the state’s junior senator can be expected to step up her re-election efforts. She needs to.

Boxer is a three term incumbent, and well known to Californians. Yet she only earns the support of 51% of registered voters. Because incumbents are inherently better known among voters than challengers, the rule of thumb is that any incumbent who scores at or below 50% of likely voters has cause for concern.

The Field Poll surveys registered voters, not likely voters. Since elections are determined by those who show up, Field’s methodology consistently oversamples Democrats and undersamples Republicans by failing to take Republicans’ higher turnout rates into account. The voting electorate next November can be expected to be more than 3% more Republican than the registered electorate. Adjusted for turnout, Boxer’s re-elect numbers are definitely under 50%.

A re-election campaign is a referendum on the incumbent, and in this case Boxer is remarkably weak. Even against unknown opponents, she can’t crack 50% of likely voters despite her 17 years in office and every advantage of incumbency.

Democrats and some in the press will look at Boxer’s performance in head-to-head matchups at this stage and claim the race is over. That’s spin, not analysis, because it fails to take into account that at this early stage the Republican challengers are naturally going to be less well known than the three term incumbent. That won’t be the case this time next year.

In the last 12 mid-term elections, the party out of power has picked up seats in Congress in 10 of them, with the size of the gains in direct, inverse proportion to the President’s approval rating. Barack Obama’s 50% approval rating today represents the fastest decline in support for almost any President in modern times. Republicans are running ahead in generic ballot tests. Republican self-identification is up.

All in all, the political indicators all point to a U.S. Senate race in California that is far more competitive than any Democrat should be comfortable with.