According to the NYT blog The Caucus:
1. Biden.The idea is that Biden's too old to run in 2016, and that will make 2016 an easier year.
2. The economy.It might improve.
3. Money. Mr. Obama is expected in some quarters to raise $1 billion for his re-election campaign, and he has no serious primary opposition, which means he will be free to aim that firepower at his Republican adversaries....
4. The Tea Party. The emergence of the Tea Party movement as a force inside the Republican Party requires potential presidential candidates to pick sides in an intraparty philosophical struggle....
5. The media glare. Candidates for president have always had to contend with scrutiny from the press. But the intense, Internet-driven political environment in 2011, when everyone has a camera phone and every offhand comment can be recorded, is enough to scare away even the most hearty of politicians....
1 & 2 BS
4 only if their views differ greatly from the TEA party.
5 they should know and be ready for the Obama media.