A Washington Post-ABC News poll shows that 60 percent of Republicans oppose a path to citizenship for illegal immigrants, even as 57 percent of all Americans -- and a similar majority of independents -- favor one.

So, what do those numbers mean for the likelihood of comprehensive immigration reform passing Congress? Since the real battle over immigration reform is likely to be in the House, where Republicans still hold a clear majority, let's look at that chamber.

According to the WaPo-ABC poll, 60 percent of Republicans nationally oppose a path to citizenship. That suggests, in your average swing district, about 60 percent of Republicans would stand opposed.

But the vast majority of Republicans don’t come from swing districts; they come from conservative-leaning districts. In fact, according to a study of the new congressional map from Fair Vote, all but 50 of the 232 House Republicans come from districts that favor Republicans by double digits (at least a 55 percent-45 percent partisanship advantage).

In other words, all but 50 House Republicans have very little to worry about in the general election but plenty to worry about in the primary. (And as their districts get more conservative, the higher the likelihood of a primary backlash against a vote for a path to citizenship.)

So, assuming that Republicans in the more conservative districts don't vote for a path to citizenship, it's up to the 50 or so Republicans who come from the swingier districts.

All but a handful of these districts favor Republicans to some degree, so it's relatively safe to assume that support for a path to citizenship in these districts is between a narrow majority and the 57 percent national figure.

These members have a calculation to make: Do I vote against something that 60 percent of my party base opposes to save myself in the primary? Or do I vote for something that a majority -- but less than 60 percent -- of all my constituents support?

Assuming all Democrats do support it, the bill would need 17 Republicans to pass. So about one-third of these 50 Republicans would need to decide that voting against comprehensive immigration reform would be a bigger liability in the general election than voting in favor of it would be in the primary. Or, they may decide that even if they think it would be a bigger liability in the primary, they are willing to take one for the team.

The math on this issue, in fact, is very similar to how it looks on an issue like gay marriage, where the GOP base stands strongly against it (67 percent opposed in a recent WaPo-ABC poll) even as a clear majority of all Americans (58 percent in the same poll) support it.

Think about how few House Republicans have announced support for gay marriage (read: two out of 232), and then consider how many of them are itching to vote for a path to citizenship.

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