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Renee Ellmers is in a highly unusual position for a House Republican: She is the only GOP incumbent facing a primary challenge centered on her support for immigration reform.

The North Carolina Republican is one of a handful of House GOP lawmakers to publicly advocate legalizing the millions of immigrants who are here illegally. Her views sparked a Republican challenge from economic commentator Frank Roche, who is skewering Ellmers for favoring amnesty.

Most observers think Ellmers a nurse and former tea party favorite is likely to win the intraparty fight on Tuesday. Still, reform advocates, particularly those on the center-right, are closely watching her race as a test case of how much the politically charged issue of immigration will matter in GOP primaries. Republicans had feared that conservatives, stoked by grass-roots anger and help from outside groups, would descend on the districts of members who sided with the reformers.

(ELECTION CENTRAL: Indiana, North Carolina and Ohio primaries)

But in a surprise to some, Ellmers fight has been the exception to the rule in this years House GOP primary contests. Despite conservative threats, the slew of anti-immigration primary challenges for the most part simply havent materialized. Of course, Democrats could still badger Republicans on immigration come November.

Filing deadlines for more than 80 percent of sitting House Republicans elapsed as of the end of April. And advocates closely tracking GOP primaries could name only Ellmers race as one where an incumbent House Republican is facing a primary precisely over his or her immigration stance.

Reps. Sam Johnson and John Carter of Texas two Republican negotiators in a bipartisan House group that painstakingly tried to negotiate a House immigration bill with a pathway to citizenship breezed through their March 4 primaries without much being made of their advocacy. Johnson walloped his challenger by 80 percent, and Carter didnt even have an opponent.

(Also on POLITICO: Little hope for Keystone vote)

The Mark Zuckerberg-backed advocacy group FWD.us argued earlier this year that only one incumbent congressional Republican lost to a primary opponent primarily because of immigration in the past decade: then-Utah Rep. Chris Cannon to Rep. Jason Chaffetz in 2008.

Its essentially a nonissue in most of these races, said Jeremy Robbins, the executive director of the Michael Bloomberg-backed Partnership for a New American Economy, which supports immigration reform. But he added: It concerns me any time that someone who is very, very good on this issue is challenged.

Read more:
The race where immigration matters