Here's a theory: Barack Obama has never in his life run against a sharp, determined and aggressive Republican opponent. Facing Mitt Romney, who is all three, is a new experience for the president.
Look at Obama's political career. He won his first election to the Illinois Senate in 1996 mainly by challenging signatures on his Democratic primary opponent's candidacy petitions and getting her kicked off the ballot.
Re-election was no problem in Obama's heavily Democratic district. The only race he would ever lose came in 2000, when he mounted a primary challenge against Democratic Rep. Bobby Rush for a seat in Congress. Republicans were not a factor.
When Obama ran for U.S. Senate in 2004, both his Democratic primary opponent and Republican general election opponent imploded when their (ugly) divorce records were made public. Obama ran for a while with no opponent at all until GOP gadfly Alan Keyes moved to Illinois to offer weak opposition. Obama won in a landslide.
In 2008, Obama faced the race of his life against Hillary Clinton in the Democratic presidential primaries. He ran a good campaign but was also aided by some inexplicable Clinton mistakes in which she failed to exploit the system by which Democratic delegates were awarded.
When the general election came around, Obama faced a Republican opponent, John McCain, who had lost a step from his GOP primary run eight years earlier, whose campaign was riven by internal turmoil, and who simply was not determined to do what it took to win.