The ad was accurate but misleading: Kingston's position is that "there should be no amnesty for any individual who has crossed the border illegally." And while Kingston gladly accepted the support of the pro-reform Chamber, Perdue had also sought the Chamber's endorsement, unsuccessfully, leading the Chamber to proclaim the attack on Kingston mere "sour grapes."

As with Cantor, plenty of other factors played a role in the Georgia runoff. (Most postmortems on the race focused on Perdue's appeal as a political outsider, and Kingston's status as a 22-year member of Congress, in a political climate seen as hostile to Washington.) But given the closeness of the result, the late-breaking amnesty attack may well have pushed Perdue over the top. If so, it would be the second time this year that Republican base voters have risen up against a candidate seen as sympathetic to the business community's desire for immigration reform.

In the past, contrary to popular belief, support for immigration reform has seldom been toxic in Republican primaries. (A notable exception came four years ago in Georgia, when Nathan Deal ran to the right on immigration on the way to winning his gubernatorial primary and the governorship.) But the current crisis on the border has inflamed the perpetual hot-button issue, particularly among the vocal minority of the Republican base for whom the only acceptable "reform" is mass deportation. And candidates like Perdue are exploiting the issue as a wedge.

That's bad for immigration reform, which was already stalled largely because of House Republicans' fear of just this sort of political backlash. And it's probably bad for the long-term prospects of the Republican Party, whose elites are convinced its future national success rests on increasing its share of the Hispanic vote—a process they believe must start with passing immigration reform. Here's a representative take from Tom Donohue, president of the (100 percent, openly pro-amnesty) Chamber of Commerce: "If the Republicans don’t do it, they shouldn’t bother to run a candidate in 2016," he said in May.

I asked Frank Sharry, the longtime immigration-reform advocate who heads America's Voice, if he saw political events like the Georgia runoff as a worrisome development. "Now that the GOP has blown the best chance in a generation to pass landmark immigration reform, I'm glad the GOP is lurching further to the right," he replied. "It will hasten their demise as a national force and lead to Democratic control of the White House and both chambers of Congress sooner. Then, and probably only then, will we get immigration reform."

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