Here’s an interesting note that I haven’t seen reported very much: of 193 superdelegates who have not made public endorsements, 30 “unpledged add-on delegates” have not even been identified yet. In addition to known superdelegates, like governors and party officials, each state gets a certain number of unpledged add-on delegates in proportion to its population - they are basically extra superdelegates.

The unpledged add-ons are chosen in different ways, depending on the state, throughout the primary season - usually at the state’s Democratic convention or by the state’s Democratic committee meeting. Of the 78.5 add-ons, 30 have not been chosen - including two from Washington, who will be chosen June 15 during the state convention.

The interesting thing is how the unpledged add-ons are selected, and by whom:

In past years, states used their extra delegates to reward elected officials, donors or labor leaders, or to achieve racial balance in their delegations. This year, the battle for the extra delegates is one of many fronts in a historic fight for the Democratic nomination.

Aides to both campaigns say they are wading into local politics to try to make sure the new delegates are amenable to their candidate.

Some state party chairmen will consult governors or senators when making their choice; others will simply pick like-minded delegates.

On May 18, Art Torres, chairman of the California Democratic party, chose his state’s five unpledged add-ons based on how the vote went in California. Clinton won 52% of the vote and was awarded three unpledged add-ons, leaving the other two for Obama.

In contrast, some state party chairmen chose unpledged add-ons based on their own preference:

That’s what Wyoming Democratic Chairman John Millin plans to do when he selects the state’s extra delegate in May. Millin, who has endorsed Obama, said he plans to choose another Obama supporter for the spot, though he hopes their votes are not decisive.

“The two votes that I get are frankly two more votes than I really want at the national convention,” Millin said. “The party as a whole needs to wrap this up soon after the primaries. I would like to see the decision made long before we get to Denver.”

And indeed, W. Patrick Goggles, an Obama supporter, was selected at the state convention on May 24.

The 30 unpledged add-ons that have not been chosen will not be enough to save Clinton. But it’s interesting to look at the process of selecting the add-ons to see how they may have affected the race. You can see the whole add-on selection schedule here at DemConWatch - it identifies the add-ons that have already been selected, how they were chosen, and who they support (if anyone). It also shows the states who have not chosen their add-ons, including big states like Pennsylvania and Texas. Each of those state’s three add-ons will be chosen on June 7.

According to my count based on DemConWatch’s information, of the add-ons who have been selected and have made endorsements, 27.5 have backed Obama and 17 have backed Clinton.

If Obama clinches the nomination today or in the near future, it will be interesting to see how the 30 remaining add-ons will be selected. Will Obama supporters be chosen in an effort to show a party united behind its candidate? Or will Clinton earn some add-ons based on the show of voting support in certain states, like Pennsylvania? And who are the two add-ones who will be chosen Washington state? Just a few more interesting factors to keep an eye on in this increasingly interesting race.