Ted Cruz and Marco Rubio need a pep talk from Herm Edwards: “You play to win the game. You play to win the game."

Trump's insistence at last Saturday's debate that George W. Bush "lied" to America in order to foment the Iraq war was important because it confirms the two big Republican fears about Trump: that he isn't actually conservative, but that he is a conspiracy-minded crank. It also places Trump in direct opposition to a figure most Republicans like very much. (To take just one example, in Quinnipiac's last poll, Trump's favorability among Republicans is +31, George W. Bush's is +67.)

One of the reasons Trump has risen is that he's the only candidate in double-digits who hasn't faced significant opposition advertising. It is inconceivable that Cruz, Rubio, and their assorted Super PACs aren't carpet bombing the airwaves in South Carolina and the other SEC states with anti-Trump "Bush lied" ads. Two weeks from now, "Bush lied" ought to be the first thing that comes to Republican voters' minds' when they think about Donald J. Trump.

Any yet so far from the campaigns . . . nothing. The Cruz campaign is hitting him on judges and social issues. Rubio sometimes dings him on eminent domain. There is no mention, anywhere, of his contention that "Bush lied."

Between them, the Cruz, Rubio (and Bush) campaigns and their Super PACs have spent more than $100 million attacking each other. And now that they've been given a 500 kiloton nuclear weapon to destroy Trump, they're refusing to use it.

Their reticence is the consequence of the reverse prisoner's dilemma they face: Each of the campaigns believes that their best chance to win the nomination is to wind up in a two-way race with Trump. So they are concentrating their resources on eliminating each other. This is not a crazy theory—Trump's favorability numbers within the party are extremely weak and both Cruz and Rubio would stand a good chance of defeating him heads-up.

On the other hand, you never win unless you play to win and the best thing for each of the rival campaigns would be wound Trump in this moment of vulnerability. Tie Trump to "Bush lied" and Cruz and Rubio will pick up Trump supporters as they drop off. (And as Stuart Stevens never tires of reminding people, Trump is sitting on the biggest pile of votes; even his attrition rate is small, the raw number of votes you can peel away from him will be large.)

But even more importantly, hitting Trump with "Bush lied" will poison the well for him in the future, keeping Carson, Bush, and Kasich voters from flowing to him as their candidates drop out. Plaster "Bush lied" all over Trump now and it will stick with him for the rest of the campaign as a piece of permanent signage—a warning that if you sign up with Trump, you're signing up for the Code Pink view of the world, too.

In short, tying Trump to "Bush lied" is a way to both bleed Trump's support now and lower his ceiling in the future.

If the other campaigns don't use this weapon against Trump now, they run the chance of it turning into a dud. Start running "Bush lied" ads in a month when it's a two-man race and it will be old news. By that point, Trump will have made 30 other ridiculous statements. And he'll be able say, Look, I said "Bush lied" a million years ago and I still won South Carolina, you loser!

Worse still, if the other candidates don't make Trump pay now, it will add to his mystique. People will believe that his support is so durable that he can survive anything—even aligning himself with Code Pink and Michael Moore.

But the truth is that it's easy to survive your mistakes in a campaign if no one makes you pay for them. If Cruz and Rubio don't nuke Trump from orbit with "Bush lied" and he somehow finds his way to the nomination, they'll be thinking not of Herm Edwards, but of Denny Green: Trump was what they thought he was. And they let him off the hook.