"I think they're remarkable," Tillis said when I asked him about the ads, noting that in addition to Hagan's campaign, liberal political-action committees have also poured money into the attacks on him. "They've spent almost $6 million meddling in a Republican primary ... and I think that that's really a testament to the fact that they don't want to face us in November, because they know we're going to defeat Kay Hagan."

The past few years' paroxysms of Republican infighting have been a boon for Democrats, who have been less and less shy about overtly exploiting division by meddling in the other party's primaries. If Tillis succeeds on Tuesday, he will have notched a victory not only against his primary opponents, but also against the left's attempts to knock him out early.

The tactic's pioneer was Harry Reid, the Senate majority leader, whose 2010 campaign brain trust thought he stood a better chance against Sharron Angle, a far-right Tea Partier with a checkered history, than against Sue Lowden, the early frontrunner backed by the state GOP establishment. The idea of Reid meddling in the Republican primary was nonetheless a sensitive enough idea that he sought to distance himself from it; it was a nominally independent political-action committee called Patriot Majority that began attacking Lowden for her "chickens for checkups" gaffe, in which she proposed that people barter poultry for health care, like in the old days.

Patriot Majority was styled to seem like a Tea Party group, with its militia-invoking name and a logo that looked like a Minuteman. But it was run by a former Reid aide and funded mostly by labor unions. The group spent $320,000 attacking Lowden at a time when Angle's poorly funded campaign was ill equipped to do so, helping Angle come out of nowhere to win the primary. Aides to Lowden grumbled about Reid's "fingerprints" on her loss. Reid denied having anything to do with it, and Patriot Majority immediately pivoted to attacking Angle.

By 2012, when Senator Claire McCaskill of Missouri borrowed the tactic, she felt safe meddling more overtly in the other side's primary race. It was her campaign, not an outside group, that spent $1 million on TV ads that purported to "attack" then-Representative Todd Akin—in terms calculated to appeal to conservative voters: The ads darkly "warned" that Akin was "a crusader against bigger government" with a "pro-family agenda." It was a neat little piece of political jujitsu, and it succeeded in helping elevate Akin. And as with Reid's bet on Angle, McCaskill's bet on Akin paid off. It was not long afterward that Akin was moved to speculate about "legitimate rape," setting off a national firestorm and securing McCaskill's easy victory.

Democrats also employed a version of this tactic in the 2012 presidential election, when the party and affiliated groups began beating up on Mitt Romney long before the GOP primary was settled. The attacks were seen as a no-lose proposition: If Romney survived the primary, he'd do so with a badly damaged public image; if he didn't survive, President Obama would get to face a candidate seen as weaker instead.