Within the Democratic Party, a quirky single issue has become the focus of opposition to primary challenger Robert F. Kennedy, Jr.: his reputation as an anti-vaxxer.

For the purpose of this discussion, let's set aside the question of whether or not the criticism is accurate. RFK Jr. denies being against vaccinations in general, says he is up-to-date on all vaccinations except for COVID-19, and claims the real problem is big pharma, not vaccines. Let's also ignore the obvious motivation of Democrats' attacks: Kennedy had the temerity to challenge Biden in the primaries, and opened strong with nearly 20% of the Democratic vote.

But why is this the anti-RFK Democrats' single issue? Why are they single-mindedly raging over the fact that he's (assuming for the sake of argument that it's true) anti-vax?

The coverage has been brutal and sharply focused. "Robert F. Kennedy Jr.," an NBC profile of the candidate begins, "is a conspiracy theorist running for president as a Democrat."