One of the biggest factors in politics is the economy. Even without knowing anything about the candidates if you know how well the economy is doing going into the election you can fairly accurately predict the outcome.
A new Gallup poll also highlights how much the economy drives opinion about the president. According to the poll, Americans who approve of President Obama’s handling of the economy are six times more likely to approve of his job performance overall than those who disapprove of how he deals with the economy. No other issue comes close to such a high level of correlation.
Other major issues such as war or significant corruption can overwhelm the economy, but for the most part the economy is the most important thing in politics. Getting that right should be the main focus of any incumbent.