I just read the Dana Milbank Washington Post article you referenced and I quote at length (emphasis mine):
If any good can come of the horror in Tucson, it will be that this becomes a McKinley moment for Sarah Palin and her chief spokesman, Glenn Beck.
One hundred and ten years ago, during another low point in the nation's political discourse, newspapers owned by William Randolph Hearst - who was angling for a presidential run in 1904 - published a pair of columns fantasizing about violence against President William McKinley.
Columnist Ambrose Bierce wrote that a bullet "is speeding here to stretch McKinley on his bier." Next, an unsigned column widely attributed to Hearst editor Arthur Brisbane declared: "If bad institutions and bad men can be got rid of only by killing, then the killing must be done." Six months later, a deranged man named Leon Czolgosz assassinated McKinley.
The killer claimed he was inspired by an anarchist, not by Hearst - but that didn't stop opponents from falsely claiming that Czolgosz had a copy of Hearst's New York Journal in his pocket when he did the deed. Secretary of State Elihu Root later accused Hearst of driving the "weak and excitable brain of Czolgosz" to murder. The outcry against Hearst's incitement - there were boycotts and a burning in effigy - dashed his presidential ambitions.
A similar, and long overdue, outcry has followed the Tucson killings. Sheriff Clarence Dupnik, who blamed the "vitriol we hear inflaming the American public" for the massacre, mentioned Palin by name and generally denounced conservative TV and radio commentary. Some in the media have fingered Beck, too.
While the accusations sometimes go too far - there's no evidence that either Palin or Beck inspired the Tucson suspect - the heat is well deserved. Both are finally being held to account for recklessly playing with violent images in a way that is bound to incite the unstable. In Beck's case, as I reported last year, it already has - repeatedly.
McKinley moment is used here to draw a potential similarity between the chain of events that eventually destroyed William Hearst's presidential ambition and the possible fallout of Palin's belligerent brand of politics. As Milbank writes, McKinley's murder was attributed by popular acclaim to Hearst's inciteful commentary. While this may have been untrue and unfair, majority thought that this served Hearst right. Milbank claims here that the Gabrielle Giffords attack will most likely be blamed on Palin's (and Beck's) violent rhetoric and would possibly put paid (BrE idiom) to whatever presidential aspiration she may have, which, in his [Milbank's] opinion, would be a good thing.
It is common in English to use "__moment" to draw a parallel between a significant event, period or phenomenon in history (personal, public, national, etc) and its modern-day counterpart. I give an original example:
'Did you see how Henry scored a handball against the Irish during the 2010 World Cup qualifiers?'
'Yeah, I did. It was a Maradona moment*. Classic!'
(*referencing soccer legend, Maradona's, famous "Hand of God" goal against England in 1986)