The Republicans face an uphill battle in tossing Obama out of the White House; the polls show Obama is likely to win by double digits next time.
So how hard is it to pick off a sitting president?
Historically, there are three ways to do it.
First -- running for reelection right after a close, controversial election: John Adams and Jefferson fought two hard-fought races and won one each, with Jefferson unseating Adams in 1800; the same happened with Adams' son and Jackson, and then Benjamin Harrison and Grover Cleveland much later. Almost all these races were very tight. Ford wasn't even elected, and then Carter beat him in a squeaker before being unseated by Reagan (and the Ayatollah). This scenario does not apply to Obama -- he crushed McCain.
Second, a party split: the GOP split in 1912, and as a result Taft not only lost, but came in third. The same thing almost happened to Truman -- the Democrats split three ways in 1948. Notwithstanding the efforts of some Blue Dogs like Ben Nelson to get in Obama's face, the Democratic party isn't going to split in 2012.
Third, economic disaster: Martin Van Buren was wiped out by a financial crash; the same happened to Hoover a century later. Bush 41 lost largely because of a recession. This is the issue Obama needs to keep an eye on: polls show that the American people blame the Republicans for the economic crisis and are willing to give Obama some time to clean it up, perhaps even as long as four years, but Obama would be much more comfortable running for reelection during a recovery.
So judging by history, it really is the economy, stupid.