Why is there a big gap in 2008 California Presidential Democratic Primary in the two most recent poll results?

Hillary Rodham Clinton leads the Democratic presidential candidates in California, with four in 10 likely primary voters saying they will support her, according to a Field Poll released Tuesday.

That is a higher level of support than she has registered in national polling or in a recent statewide poll by the Public Policy Institute of California, which found her leading with 35 percent.

Chris Bowers has started a new page on MyDD dedicated to the Inflated Clinton Poll Theory and his recent calculations of national polls are in line with what we are seeing in California.

Bowers examines whether Clinton performs worse as poll samples are tightened. Since Field Poll only interviewed Registered Voters, here is the situation at that level.

Bowers summarizes why this debate is important:

Right now, this is still just a theory. However, it is an important theory to test, because accurate reports on who is currently ahead in the race for the Democratic nomination, and how much that person is ahead by, are crucial to developing an informed Democratic primary and caucus electorate. Whether or not we like it, and whether or not we think it should, information of this sort has an impact on the nomination campaign. As such, it would be a disservice to the Democratic primary and caucus electorate if we did not work to make certain they had accurate information on who is winning, and by how much that person is winning. I imagine there are quite a few Republicans out there who feel the same way about their party.

One professional pollster (read through to his comment) suggests that California may be particularity prone to this which could mean Hillary may not even have a lead:

Among the general election voters who claimed they would vote in the Dem primary, Clinton scored eight points higher than among the past primary voters.