Rather than musing about the different ways that the campaign could potentially end, it is probably far more productive to figure out why the nomination campaign is not ending. Once we figure out why the campaign is continuing, we can then work to remove those causes and actually bring the campaign to an end. So, in the spirit of being productive, in the extended entry I list the six main reasons why the nomination campaign continues:
Debates: As long as the Obama campaign is agreeing to debates, and as long as debates are scheduled, then they are actually providing ample justification for the nomination campaign to continue. After all, if the nomination campaign really was a foregone conclusion, then why is the supposed presumptive nominee debating other Democrats? In order to end the nomination campaign, the Obama campaign must reject all further Democratic debates. The breaking news that the North Carolina debate is canceled is a good first step.
Clinton Attacks: As long as the Obama campaign is still attacking Clinton via press releases, emails, television ads, or any other campaign dispatches, once again it is providing all the justification necessary for the nomination campaign to continue. After all, if the nomination campaign really was a foregone conclusion, then why is the supposed presumptive nominee attacking other Democrats? In order to end the nomination campaign, the Obama campaign must stop attacking Clinton immediately. And for that matter, we need to stop attacking Clinton, too. It helps keep her relevant.
States where Clinton leads: As long as there are upcoming primary states where Clinton holds a lead in the polls, it is pretty difficult to claim that no one should pay attention to the Democratic nomination campaign. Right now, Clinton holds leads in Pennsylvania (April 22nd), West Virginia (May 13th), Kentucky (May 20th) and Puerto Rico (June 1st). Until there are no more states where Clinton leads in the polls, it is pretty difficult to argue that the nomination campaign is over. Even though Obama has a large and nearly insurmountable delegate advantage, psychologically the campaign does not seem to be over if Clinton is still winning states.
This was also the case on the Republican side. Because of his victories on February 5th and February 9th, Mike Huckabee's campaign was not dismissed in the national media until after he lost the Virginia primary on February 12th. However, once no more potential Huckabee victories were on the horizon, there was no reason to keep following his campaign. The same can be true of the Democratic contest.
Clinton still raising significant sums of money: The Clinton campaign may be in the red, but it still raised $20M in March. As long as it keeps raising money at that rate, it will be able to keep a large operation running. However, if at any point it is forced to start cutting significant staff and / or end paid media spending, that will spell the end pretty quickly. So, Clinton's donors are also keeping the nomination campaign alive.
Uncertainty: Unlike the Obama campaign, which posts a running delegate count and magic number meter on its campaign website, the Clinton campaign is working to sow as much doubt in the outcome as possible. In addition to not making an public delegate counts, the Clinton campaign also regularly sends out missives on Michigan and Florida, superdelegates, and about how even pledged delegates can flip sides. Increased clarity in delegate counts among major news organizations, and increased clarity on the 55 "uncommitted" delegates for Michigan (all but one of which now appear to actually be Obama delegates), will resolve this issue. The key is cutting through the uncertainty, and making a clear case on where the delegate count stands.
Democratic Divided Media Narrative: Above all else, the extensive media focus on the Democratic nomination campaign stems from the deep, longstanding, "Democrats divided" media narrative. After decades of writing thousands of variations on this theme, an endless, inconclusive nomination campaign must be the ultimate dream come true for the established, American political news media. There may not be much we can do about this in such a short time, apart from starting to attack the media for putting so much focus on the nomination campaign.
In summary, here is what needs to be done in order to end the nomination campaign:
The Obama campaign must reject all future debates with Clinton, and also stop attacking her. It needs to start acting like Obama is the presumptive nominee if we except the country to think of him that way.
The media needs to be pushed to provide clarity on the delegate situation, and to put more focus on the general election. They need to be attacked for their clear desire to pursue the "Democrats divided" narrative indefinitely.
Voting either needs to end, or Obama needs to take the lead in all remaining states and territories to vote. The campaign won't end as long as Clinton keeps winning a few states.
Clinton's fundraising needs to dry up. The campaign won't end if she still has enough money to maintain her current level of operations.
Apart from actually reaching the magic number to clinch the nomination, those are the concrete steps that can be taken to end the nomination campaign. All of this is far more productive than demanding that Clinton drop out. Such demands not only won't work, but cause Clinton to appear sympathetic for being attacked and principled for continuing to fight against long odds. Further, such demands cause those leveling them to look whiny and afraid, not to mention with a sense of entitlement.
If you want to end the nomination campaign, then work to remove the underlying causes that keep the nomination campaign going.