JAMES TARANTO: Cantor Defeats Truman: The political surprise of the year.

“Eric Cantor’s margin of victory Tuesday could have national GOP repercussions,” according to the subject line of a self-promotional email we received from a political writer last night. It was an update of an earlier analysis, and the text correctly noted that the House majority leader had lost. Half an hour later, the writer, whom we won’t name because we’re a nice guy, sent out a new email with a “corrected subject line,” referring to Cantor’s “primary loss.”

The error was consistent with much of the pre-election analysis. “Challenger David Brat looks almost certain to lose, but the margin may say something about the potency of the issue,” blogged The Wall Street Journal’s Laura Meckler an hour before polls closed.

“The issue” is immigration, on which Cantor had staked out a middle-of-the-road position; Brat accused him of backing “amnesty.” National Review’s Joel Gehrke reports that Obama aide Dan Pfeiffer “rushed to assure House Republicans” that “Cantor didn’t actually lose because of his gestures toward Democrats on immigration reform.”

Tweeted Pfeiffer: “Cantor’s problem wasn’t his position on immigration reform, it was his lack of a position. [South Carolina’s Sen. Lindsey] Graham wrote and passed a bill and is winning big.” Graham avoided a runoff by taking 59% of the vote in a crowded field.

National Journal’s Josh Kraushaar offers a different explanation for Graham’s victory: a combination of “assiduous attention to constituent service, paying favors to his South Carolina Republican colleagues, an early and relentless focus on fundraising, and a hawkish record on foreign policy that conservatives in the military-rich state rallied behind.” Whatever the merits of the argument, one suspects Republicans will be wary of taking advice from Pfeiffer.

In a postelection report, the Journal’s Kristina Peterson and Janet Hook described Cantor’s defeat as “overturning the building narrative that Republican Party leaders and allied business groups had trampled the GOP’s tea-party wing.” A variation of that narrative, which this column put forth last month, was that the GOP had assimilated the Tea Party.

One might say this is the exception that proves the rule, but it’s actually the second exception. A week earlier, challenger Chris McDaniel narrowly outpolled Sen. Thad Cochran of Mississippi in that state’s Republican primary.

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