Why would it be OK for non-Democrats to pick the Democratic candidate?
Over at the Perfect World, Cal Lanier crunches the numbers and finds that Obama, despite being ahead among pledged delegates, has fewer total votes among people who identify themselves as Democrats. (He has 7,392,809 votes; Clinton has 8,229,063.) That gives Clinton as lead with 52 percent of Democrats. Lanier also breaks the numbers down by race and points out that Obama has won white Democrats in only two states: New Mexico and Illinois.
The numbers are hardly perfect. They rely on CNN and MSNBC exit polls, which are inherently rough. (Extrapolating those percentages to estimate exact numbers of voters is going to compound margins of error.) And because caucuses report delegates, not individual turnout, those stats are going to be a little murky, too. I'd also dispute their inclusion of Florida and Michigan in the count. But Clinton’s lead is still large enough to be significant.
And to add insult to injury, we've got those pesky "Democrat for a Day" YouTubes and brochures that keep turning up. It seems to me that it's one thing for individual Republicans to make their own choice to turn away from the dark side, all on their own. But isn't it another thing for a campaign to encourage Republican crossovers in the hope that the tactic will give them their margin of victory? Doesn't that disenfranchise regular, loyal Democrats?
NOTE Thanks to Paul in comments for the Slate link.