Here's Jeb Bush's latest ad in New Hampshire:

It's a good one -- certainly one of the better ones to come out of Bush's campaign and/or super PAC in this presidential campaign. And the reason it's good is simple: Bush, in his own words, goes right at Republican front-runner Donald Trump for the latter's comments about a disabled reporter.

"When anybody, anybody, disparages people with disabilities ... it sets me off," Bush says in the ad. "That's why I called him a jerk. What kind of person would you want in the presidency that does that?"

Pointed. Angry (or at least as angry as Bush gets.) Effective. Also, very, very late.

The episode that Bush's ad refers to -- in which Trump mocks New York Times reporter Serge Kovaleski -- happened in late November. It was right around Thanksgiving and, even despite that holiday checking-out by many people, still received a lot of attention.

So why is Bush only now going after Trump for it? I think the answer is that it has taken Bush a very long time to fully grasp both the strength of Trump and his own dire circumstances in this race.

For months, Bush ignored Trump -- hoping he would simply go away. And even when it became clear that Trump was here to stay, Bush resisted trying to match the real estate mogul punch for punch. Now, however, Bush realizes he has no choice but to go directly after Trump in the bluntest terms possible. No other attack has the potential to succeed.

It seems unlikely to me that Bush's amped-up hit on Trump's character will deeply wound the billionaire businessman. (Nothing else has.) New Hampshire polling continues to show Trump with a wide lead over a collection of establishment rivals.

Here's my guess at Bush's calculation for running this ad now. He and his team recognize that the end game of the Republican nomination fight will be Trump and a "not Trump" candidate. And that the best way to be the "not Trump" candidate is to be seen as the one willing to take the fight to him most aggressively.

That's a savvy way to look at the field. Of course, it was also true a month or two ago -- when Bush probably should have first put this ad on TV.