In his fifth year, George W. Bush's poll numbers sank, but Republicans stayed loyal. In President Obama's fifth year, the overall polls have looked similar — thanks in large part to Democratic defections.

The comparison between Obama and Bush — or between Obama and every other president — is a natural side effect of our reaching the end of the first year of the president's second term. On Thursday, Politico drew a direct comparison between the 43rd and 44th presidents, outlining the various ways in which their fifth years have been similar. "No one disputes that Bush and Obama are very different presidents confronted by very different circumstances," Politico writes.

For Bush, the problems in his second term resulted from a plethora of issues, including his handling of the wars abroad, his controversial plan to privatize Social Security, and his disastrous nomination of Harriet Miers to the Supreme Court. For the current president, the problems are resulting almost exclusively from the bungled introduction of Obamacare.

But the polls are so similar! That's the real genesis of the introspection. Their presidencies look so similar down to the numbers.

Here's what the weekly approval ratings have looked like for Obama (blue) and Bush (red) for each one's fifth year, via Gallup. Very similar in slope, although Bush's tenure saw more volatility. (There are fewer data points for Bush in Gallup's data.)

What's interesting is when you compare how those approval ratings have compared to party support. In each of the following graphs, the top line is the difference in approval between the president's own party and the opposition; the bottom, thicker line is overall approval.

Bush

Obama

The trend is subtle but noticeable. In Bush's graph the top line jumps up and down, but remains steady. In Obama's, the line slowly declines. In other words, Bush's Republican support remained consistently distant from his Democratic support over the course of his fifth year. Obama's Democratic support, however, has become closer to his Republican support. The graph at right shows the difference clearly: the average overall split between party approvals is about the same for Bush and Obama's fifth years. But while the average difference for the first and second halves of year is about the same for Bush, the gap was much wider for Obama in the earlier part of 2013.

There are two ways that could have happened for Obama: Democratic support could have stayed flat and Republican approval risen, or Republican support could have stayed consistent and Democratic support fallen. Which was it for Obama? The first Gallup poll this year shows 20 percent higher approval among Democrats than the most recent. More than three-quarters of Democrats still approve of Obama's job performance, but the decline in his overall approval is largely because that approval has dropped. (Independents, for both Obama and Bush, have been about the same.)

That's why Obama's and Bush's fifth years really aren't that similar, besides topline polling numbers. Obama is losing support from his base where Bush kept his. The bad news for the sitting president is that Bush didn't have much more ground to lose with independents and Democrats. Obama has a lot more room to drop within his own party.