In my opinion, the worst possible scenario for the 2012 election is for Obama to win over 50% of the popular vote while Romney wins the Electoral Vote.
In such a case there will be a shrill call to arms claiming that the Republicans twarted democracy and that the the Romney election is an affront to the world spirit. Winning 50% + of the vote would set up Obama for a run in 2016 on the grounds that Romney was an illegitimate president. Romney is very likely to lose if Obama ran in 2016.
If there was a strong "Split the Vote" campaign and a large number of undecided voters realized that splitting a vote would force both parties to change, the split vote campaign would deny Obama the majority.
Imagine that Obama got 48% of the vote and Romney got 46% of the vote but won the electoral college with Libertarians getting 3% of the vote. The majority of people are clearly against the Obama regime. The claims that Obama was the true victor of the campaign diminishes. There would still be protests in the street, but the protests would not resonate.
More importantly, the inability to get a majority would reduce Obama's chances of getting the 2016 Democratic nomination.
If a split vote resulted in an election with Obama receiving less than a majority, then he would be an embattled lame duck without a clear mandate. Remember how the Republican party completely fell apart in the lame duck years of Bush?