No matter what hundreds of posters on blogs across the web are saying, the democratic primary is as good as over. But among all of the speculation of what went wrong and what went right, people have a tough time agreeing on what the critical moments in the race were. I believe that we can look to the super delegates to identify those critical moments. Without going into details of why they are so important, the Potomac primaries and the May 6th primaries are clearly the two most important moments for Obama by this metric. March 4th was a significant blow to Obama despite not hurting his delegate lead significantly, and as it turns out, Pennsylvania was not that important.
For the sake of honesty, I posted a similar post on my blog three weeks ago, but as no one has ever visited my blog (seriously zero visitors since that post) I figured I would edit, update, and change it and put it here. Enjoy!
In 2010 or 2014, when the next crop of presidential candidates are deciding their strategies, they will look back to past races, and I believe this race will be one of the most studied. This race has been long and ardous, but history will decide on a point when it was won. I believe that we can already see that history (however fresh it may be) by looking at the movements of the super-delegates.
We know the media's perspective of the race. Hillary was the inevitable nominee. After Iowa, Hillary was dead, then she cried and won New Hampshire. Then she won Nevada and he won South Carolina. Then Super Tuesday. Obama wins 14 states to Hillary's 10, but only beats her by 18 delegates (853-835). Through all of this the super-delegates remained static. 
Refer to the chart to the left. I put this together using data from demconwatch.blogspot.com , an excellent super-delegate tracker page, and they get full credit for the data. Then the Potomac Primaries. Now that changed things. Obama has an 11 state winning streak. You can see in the chart Hillary bleeds her super-delegate lead at this point.
Then Hillary wins Ohio, Rhode Island, and the Texas primary while Obama wins Vermont and the Texas caucus on March 4th. This did little to change Obama's delegate lead. Did it slow his momentum? Hard to say since the next contest wasn't until after 6 weeks of scorched earth campaigning in Pennsylvania on April 22nd, but you can see very clearly that the number of delegates moving to him over Hillary stopped entirely. So what March 4th (or Super Tuesday 2 as the joke of a news station Fox was calling it at the time) did was stop Hillary from hemorrhaging her super-delegate lead.
The next big race was Pennsylvania, this was going to be a game changer. Hillary was originally expected to win by 20 points, Obama made huge grounds, everyone still expects Hillary to win but by 2-3 points. She wins by 9. Was it a game changer? Absolutely not. The super-delegates do not move one way or another in any appreciable measure.
The next race is two weeks later, North Carolina and Indiana. Obama is expected to win NC huge and Hillary was expected to have a reasonable win in IN. This was the last big day left in the primary calendar. Obama wins big (14 points) in NC and in Indiana he only looses by 1.4%. At this point there are 217 pledged delegates remaining and Hillary needs 333 to get the nomination. With no clear path to the nomination at this point the dam broke and the super-delegates flow en masse to Obama.
So were NC and IN game changers? Yes and No. On one hand Hillary lost her delegate pickup from PA so we didn't move in delegates at all. In that sense it was not a game changer. However, at this point, the democratic establishment decided it was over and the super-delegates recognized the Clinton campaign as a sinking ship and started to abandon her. Since then Obama has picked up 73.5 super-delegates to Hillary's 11.5 (Hillary has actually had more endorsements since then, but she also had a number who had previously endorsed her defect to Obama, so they count as net losses). It that sense, it changed the game by effectively taking away her last real shot at winning the nomination (the super-delegates over-riding the delegate vote).
Now if you listen to the pundits, we learned one important piece of information from Ohio and later Pennsylvania. Obama has a problem with white working class voters. The next two races would prove to be the worst of Obama's campaign, West Virginia on May 13 and Kentucky on May 20. Both are low on black voters, low on affluent voters, and very low on educated voters. In both Obama suffered 30+ point losses. Fortunately for him, WV was so small Hillary's delegate pickup was insignificant, and Kentucky fell on the same day as a much better state for Obama, Oregon.
His commanding win in Oregon (18%) did not make up for his loss in KT (35%), but there are not many whiter, poorer states than Oregon. The real effect this win had was showing that Obama did not have a white working class problem, he had an Appalachian problem (a problem I suspect stems from Obama being seen as not very labor friendly, rather than anything else). Obama was expected to lose WV by a large margin, and as such, the results had almost no impact on the super-delegates as a whole. If you look at the graph, there is a change in slope after KT and OR. One may be tempted to wonder if the results from Kentucky concerned supers with Obama's chances with the above mentioned white working class voters. That is a possibility. I will posit another possibility. I believe that the super delegates that are remaining are looking at the calendar and realizing that there are only two states and a territory that has no say in the general left in the next two weeks. Get Obama close enough that he can pass the magic number during these contests rather than pushing him past by themselves. I am not a super delegated, but I imagine that nobody would want to be delegate number 2025, the kingmaker if you will.
Tomorrow (Saturday), the DNC Rules and Bylaws Committee will be meeting to decide how to deal with Florida and Michigan. The results of Saturday will determine if Hillary drops out next week or stays in until the convention. But regardless of how she exits, I believe that when analysts look back on this race in 2010 and later, they will see the Potomac primaries and the NC and IN primaries as the two key days. It may be that May 6th was the day Obama won the nomination.