WASHINGTON — Writing in the Brookings Institution’s FixGov blog last week, political scientist Christopher Parker pondered House Republicans’ stubborn refusal to back immigration reform, despite support in the Senate and across wide swaths of the conservative commentariat. He surmises that House Republicans are balking because they “represent constituencies haunted by anxiety associated with the perception that they’re ‘losing their country’ to immigrants from south of the border.”

Recent polling backs this up. Significant numbers of conservatives, and white Americans in general, admit to feeling discomfort at the prospect of a non-majority white America. These views are even stronger among Tea Party-aligned conservatives. According to Parker’s polling, nearly two-thirds of Tea Party conservatives want to eliminate birthright citizenship, and 82 percent of Tea Partiers say they feel “anxious or fearful” about undocumented immigrants.

Another factor behind Republican recalcitrance on immigration and similar issues is the simple racial math underlying many House congressional districts. According to U.S. Census data, only 13 out of 234 Republican-held districts are majority-minority (that is, districts where white non-Hispanics make up less than 50 percent of the population). That’s about 5 percent of all Republican districts. In contrast, fully 49 percent of Democrat-held districts are majority-minority.

You can see how this looks in the accompanying chart, which plots one thin bar for every congressional district in the U.S., sorted by the white non-Hispanic share of the district population, and colored according to whether a Democrat or Republican holds the seat.

On the left side of the chart are districts with the lowest white non-Hispanic population share. These districts are overwhelmingly Democratic. The least-white district in the United States is New York’s 15th, which lies within the Bronx and is held by Democrat Jose Serrano. In terms of ranking by non-white population share a Republican district doesn’t show up until number 21 on the list — that would be Florida’s 27th, a majority Hispanic district with a large Cuban population, held by Ileana Ros-Lehtinen.

Republicans, on the other hand, are better represented on the right side of the chart. The whitest district in the nation (at 96.2 percent) is Kentucky’s 5th, represented by Republican Hal Rogers. There are a fair number of Democrat-held districts over here too — seats in highly liberal but overwhelmingly white New England states like Maine and Vermont, as well as some seats in West Virginia and the Northern Great Lakes region.