Ted Cruz had a teachable moment in Iowa, although he himself will learn nothing from it. A voter told Mr. Cruz the story of his brother-in-law, a barber who had never been able to afford health insurance. He finally got insurance thanks to Obamacare — and discovered that it was too late. He had terminal cancer, and nothing could be done.

The voter asked how the candidate would replace the law that might have saved his brother-in-law if it had been in effect earlier. Needless to say, all he got was boilerplate about government regulations and the usual false claims that Obamacare has destroyed “millions of jobs” and caused premiums to “skyrocket.”

For the record, job growth since the Affordable Care Act went fully into effect has been the best since the 1990s, and health costs have risen much more slowly than before.

So Mr. Cruz has a truth problem. But what else can we learn from this encounter? That the Affordable Care Act is already doing enormous good. It came too late to save one man’s life, but it will surely save many others. Why, then, do we hear not just conservatives but also many progressives trashing President Obama’s biggest policy achievement?