5) The European debt crisis could sink the world economy
Here the president is totally at the mercy of events. His State Department and Treasury secretary can do their best to negotiate a European rescue package, but if the negotiations fail or the package itself fails, the world economy could slip back into a severe recession with devastating consequences for the Democrats.
6) The Middle East could erupt in war and sink the world economy
Once again, the president is at the mercy of events here, as a war with Iran could shut down the production and shipment of oil, sending the price over $300 per barrel and gas prices to almost $10 per gallon. Previous oil shocks in 1973, 1979, 1990, and 2008 all led to recessions and defeat for the incumbent party in the next election.
7) The turnout among minorities and the young could fall off
President Obama inspired a record black turnout and carried the under-30 vote by 2-1, thus helping him carry normally Republican states like Virginia, North Carolina, and Indiana, plus the key swing states of Ohio, Florida, and Colorado. That big Democratic surge was invisible in 2010. A lower turnout from the Democratic base could be fatal in a close election. Ask the supporters of Hubert Humphrey and Gore about that.
8. An independent candidate could have unforeseen consequences
John Anderson in 1980 cost Jimmy Carter at least a half-dozen states; Ross Perot destroyed the first President Bush in GOP strongholds like San Diego, Dallas, and Phoenix plus suburbia everywhere in 1992; Ralph Nader cost Al Gore Florida and New Hampshire in 2000. Who knows what effect a possible Nader, Ron Paul, Donald Trump, Mike Bloomberg, or other independent run would have?