People who are writing Mitt Romney's political obituary right now have a short memory.

In fact, the last two presidents, before winning their first terms, both trailed in the post-convention phase of the race.

While all of us remember President Obama's big victory in 2008, many of us forget that, in early September, he actually trailed in most national polls.

In much the same way that Obama rose in early September, so did Sen. John McCain (R-Ariz.) For a one-week period between Sept. 5 and Sept. 12, Obama led in just two of just 12 national polls, according to Real Clear Politics, while other polls showed McCain up three, four, five and even 10 points.

Here's a handy chart from Gallup showing that McCain bump, which eroded just as quickly:

Similarly, in 2000, Gallup polling in early-to-mid-September showed Al Gore leading George W. Bush by as many as 10 points.

Just weeks later, Bush took a double-digit lead of his own, and then the race wound up in a virtual tie.

(One difference between this year and 2000/2008: Bush and Obama led for much of the year even though they trailed in September. Romney, in contrast, has trailed for the vast majority of the year.)

The lesson from all of this: Things change. And they can change quickly.

The political events of one week can consume the news media so wholly that the impact of a major news story will often be overestimated. 

Plenty of things will happen over the next 45 days. Obama has a good start -- especially in the swing states -- but it's totally reasonable to think this race will shift again. It almost always does.