Run it now.
The moment is ripe. Like you should've done with the debt ceiling and default fiasco, a moment when you could've destroyed all Republican public credibility in one fell swoop. This time it's the nuclear arms deal with Iran about which the Republicans have lustily overreached. The stakes are even more dire, and the political advantage is there for the taking.
There's also the small matter that, beyond its domestic benefit as a political move, running today's version of that ad could seal the peace deal with Iran and avert more violent conflict and suffering in the Middle East and beyond, and it might just persuade the American public to assert its better, loving, maybe-we-will-all-live-happily-ever-after-someday side. It could, by fortuitous chance, even be the thing that tips this teetering global civilization back into having a fighting (ahem) chance at long-term viability. Just a thought.
And yes, it's true that it wouldn't have been nearly as convincing for Johnson to run that ad in 1968 as it was when he did it against Goldwater in 1964. But, somewhat bizarrely, today the Democratic Party and President Obama could once more lay claim to that grand ideal, the hope that we can make love and life prevail over hate and death. And today, half a century later, the portrayal of the Republican Party's ghastly, retrograde vision for our world is more accurate than ever.
Just think about what it would mean for the Democratic Party to run that ad again against the Republicans today.