CBS hits the ground first among general-election polls after the official end of the primaries, and no one should be surprised to see Barack Obama leading John McCain. The margin will surprise a few people, as McCain remains in throwing distance at six points back despite the usual skewing of the sample by CBS. McCain only trails by six, and Obama has a big problem among Hillary Clinton supporters:

Presumptive Democratic nominee Barack Obama holds a six point lead over his Republican counterpart John McCain, a new CBS News poll finds. Obama leads McCain 48 percent to 42 percent among registered voters, with 6 percent of respondents undecided.

The poll contains troubling signs for Obama as he looks to mobilize the Democratic Party behind him following his long and sometimes bitter battle with Hillary Clinton for the Democratic nomination, however.

Twelve percent of Democrats say they will support McCain in the general election. That’s higher than the 8 percent of Democrats who defected to President Bush in 2004. Nearly a quarter of Clinton supporters say they will back McCain instead of Obama in the general election.

McCain leads Obama by 8 points among registered independent voters, considered a key voting block in November. The Arizona senator leads Obama 46 percent to 38 percent, with 11 percent of respondents undecided.

That last number should have tipped CBS that their sampling was off … again. If McCain leads among independent voters by eight points, it makes little sense that he would trail overall by six. When adding in the 12% of Democrats who plan to defect to McCain, the overall number appears impossible.

Of course, with the CBS poll, nothing is ever impossible; it’s just a matter of juggling the numbers. In their original sample, they surveyed 930 registered voters. The registration comprised 27% Republicans, 40.5% Democrats, and 32.5% independents. CBS thought those numbers looked incorrect, and so “weighted” the numbers to get a better representation of how CBS sees the electorate. The final breakdowns?

  • Republicans – 25.5%
  • Democrats – 41.5%
  • Independents – 33%

CBS expanded the gap between Democrats and Republicans from 13.5% to 16%, while edging up independents slightly. Small wonder, then, that the Democrat leads in this poll. And that is among registered voters, not the far-more-predictive likely-voter samples, which tend to disfavor Democrats.

Even with that boost, the numbers do not look promising for Obama. Only 63% of Hillary Clinton’s supporters plan on voting for Barack Obama, while 22% plan on voting for John McCain. Another eight percent won’t vote at all, and only 7% claim to be undecided or don’t know how they plan to vote in the fall. That looks like a solid bloc of Democrats for McCain, especially given the option to choose Undecided and Won’t Vote instead of a binary McCain/Obama option. In contrast, John Kerry held all but 8% of Democrats in 2004 and still lost the race to George Bush — and Bush isn’t running this year.