I am no Kerry fan, but I have always felt that Dean's Achilles' Heal was his support for middle-class tax increases. Kerry drove that home in Iowa via effective ads. If Dean ever started to rise again, the same kind of ad would simply knock him down again.
From today's NYT:
"[In Iowa] the [Kerry] campaign was broadcasting a series of emotional and, its polling showed, effective ads. In one in Iowa, a widowed office worker named Elizabeth Hendrix recounted her struggle to raise four boys on $28,000 a year, and praised Mr. Kerry's proposal to roll back only the Bush administration's tax cuts on the wealthiest Americans, not everyone, as Dr. Dean and Representative Richard A. Gephardt of Missouri were proposing. Looking straight into the camera, Ms. Hendrix declared, "John Kerry understands what's going on in my life."