Measured as a share of the popular vote, Bush beat Kerry by just 2.9 percentage points: 51% to 48.1%. That's the smallest margin of victory for a reelected president since 1828.
The only previous incumbent who won a second term nearly so narrowly was Democrat Woodrow Wilson: In 1916, he beat Republican Charles E. Hughes by 3.1 percentage points. Apart from Truman in 1948 (whose winning margin was 4.5 percentage points), every other president elected to a second term since 1832 has at least doubled the margin that Bush had over Kerry.
In that 1916 election, Wilson won only 277 out of 531 electoral college votes. That makes Wilson the only reelected president in the past century who won with fewer electoral college votes than Bush's 286.
Measured another way, Bush won 53% of the 538 electoral college votes available this year. Of all the chief executives reelected since the 12th Amendment separated the vote for president and vice president — a group that stretches back to Thomas Jefferson in 1804 — only Wilson (at 52%) won a smaller share of the available electoral college votes. In the end, for all his gains, Bush carried just two states that he lost last time.