In New York’s 15th Congressional District, the Republican Party is functionally extinct. The district is part of the Bronx – an area that hasn’t voted for a Republican presidential candidate since Calvin Coolidge in 1924 – and its congressman, Democratic Rep. Jose Serrano, didn’t even draw a Republican challenger during the GOP wave of 2014.

But a handful of Republican voters in extremely blue districts like New York’s 15th just might decide the GOP presidential nomination. Specifically, delegate-allocation rules give relatively small contingents of Republican voters in highly Democratic enclaves such as New York City, Los Angeles and Philadelphia a huge amount of leverage, and the outcome of these unpredictable contests could tip the nomination to or away from Donald Trump.

Two seemingly innocuous details buried in the Republican rules give blue district voters a large amount of leverage. First, each congressional district -- no matter how big, small, liberal, conservative, urban or rural -- gets three delegates. Second, the RNC let states set many of their own rules, so in some states (e.g. California and New York) those three delegates are allotted according to the result in that district rather than statewide.

That means that heavily Republican districts award the same number of delegates as districts that are packed with Democrats. This phenomenon has been documented in a number of other articles, but we can get a better sense of how numerous, important and unpredictable these districts are by laying out some numbers.

This table shows the 30 most Democratic districts in the country, according to the 2012 presidential vote (as calculated by David Nir of Daily Kos Elections). It’s sorted from lowest to highest Republican share of the vote, and it includes some demographic details (from the American Community Survey). The final column shows the Republican primary winner in that district (from official results, David Leip's Election Atlas or Ace of Spades’ J. Miles Coleman). Districts that haven’t voted yet are shown in bold.

A quick glance at the bolded rows shows that the most-Democratic districts -- many of which are majority-minority districts New York, Philadelphia and Los Angeles -- still haven’t voted. We could try to use what we know about the demographics of districts that have already voted -- e.g. that Trump does better in areas with lower incomes, lower levels of education, etc. -- to project the results of these contests. But the extreme scarcity of Republicans in these places makes that sort of forecasting difficult.

Specifically, we don’t have a good read on who the Republicans in these districts are, how regional and local culture will shape the preferences of these voters, whether not-so-strong blue state GOP organizations have any influence on these voters, or if campaign effects (e.g. an extra-strong mobilization effort from the Ted Cruz campaign) could flip a district away from a candidate who is a more natural fit. Additionally, blue district Republicans who have already voted often had to choose between four or five candidates rather than three, so that difference makes it more difficult to use results from those districts to predict future results.

We can (and should) try to eliminate these uncertainties through good reporting and rigorous analysis, but we can’t eliminate the root of the problem: that we haven’t yet seen districts quite like the upcoming ones in this GOP primary.

Additionally, seeing results from some of these districts will only be of limited help in predicting the results of others. For example, a number of majority African-American districts in New York City will vote on April 19, and we’ll get good data from that. But that data might not be helpful for predicting how Republicans in heavily Hispanic areas of Los Angeles will vote on June 7.

This leaves us with a small- to medium-sized state’s worth of extremely blue, functionally unpredictable congressional districts -- adding even more uncertainty to an already tumultuous election.