Why would Mark Zuckerberg, the Facebook billionaire born and raised in New York, educated in Massachusetts, and now based in California, have any particular interest in the politics of Iowa's rural 4th Congressional District? Because the area is home to Republican Rep. Steve King, one of the most vocal, and certainly controversial, opponents of the immigration reform bill Zuckerberg and fellow Silicon Valley moguls want to pass.

Zuckerberg's pro-reform group, FWD.us, has produced an ad attacking King for his opposition to the ENLIST Act, a measure that would allow some illegal immigrants to win legal status by joining the U.S. military. "As soon as they raise their hand and say 'I'm unlawfully present in the United States,' we're not going to take your oath into the military, but we're going to take your deposition and we have a bus for you to Tijuana," King told Breitbart News this month.

The FWD.us ad, which will run in Iowa, says: "Instead of supporting our military, Steve King, a Republican member of Congress, insults the brave soldiers who are immigrants and those who would proudly serve. Instead of supporting immigrants who want to serve, he'd deport them. Steve King’s attacks on American soldiers and the military is [sic] wrong."

King is in a safe, conservative, Republican district and is expected to win re-election easily this year. The ad is unlikely to do him much harm at home. And National Journal reports that, "Although FWD.us is going hard after King, the organization doesn't have plans to utilize similar messaging against other elected officials."

So why air the ad at all? As a warning to other lawmakers who would oppose FWD.us. "This sends a message," says a GOP Hill aide who opposes Gang of Eight-style reform. "Do you want this headache? Do you want this amount of trouble?" A lawmaker who gets on the wrong side of the FWD.us billionaires might find himself or herself receiving the King treatment. "They're trying to push a group of Republicans out of the party," the aide says, "and, in a left-wing way, if they can't beat them, then try to delegitimize them."