These 18 states have in common that they all have at least 4 EVs and that they are on an upward spiral for more EVs, not less. For all except a couple (being Virginia & Tennessee), they're right now on an all time high peak of electoral votes; and for most of these states, they're expected to gain even more EVs in the near or midrange term future.
Together these 18 states have an impressive 260 EVs already, just being 10 EVs short of an overall majority. With a little luck, yet not too much I presume either, they might have secured those extra 10 EVs already by the 2020 census, in just six more years. That would make them truely worth the labeling "the rising electorate of states", and make them even more all important come the 2024 battle of giants.
As you can easily see, all of these states are part of the rising regions of the US, that is either the south or the west - or both. 11 of the states - including the two biggest, California & Texas - are part of the west, while the remaining 7 states are part of the south or southeast (6 of the 7 even making up the Atlantic South), Florida being the most prominent (and by the 2016 showdown already more populous than New York, making it ranking 3rd, an event that will probably unfold already this year).
When we look at which parties are currently dominating these 18 states, we'll see that 10 of them voted for Obama in 2012 (and 11 in 2008), while 8 voted for Romney. When we then count the electoral votes given to each candidate in 2012, we see that states making up 150 EVs preferred the Democratic candidate, while states counting for just 110 EVs preferred the Republican candidate, which clearly gives future Democratic candidates the edge and upper hand on this rising electorate of states. What makes it even worse for future Republican candidates is that a majority of these states are in fact trending Democratic and have been doing so over the span of the past number of presidential elections (a trend that is only expected to grow stronger due to unequivocal demographic trends). 4 of the 8 Republican states in the rising electorate of states - North Carolina, Arizona, Georgia and Texas - are widely expected to turn purple, and eventually (perhaps even) blue in the near or not so distant future. If such a development was to materialize, it could leave future Democratic candidates with the hold on no less but 14 of the all important 18 states of the rising electorate of states, making a future revolution within the Republican party perhaps inevitable in order to survive and stay relevant for voters on a broader scale.
For more on the changing allocation of EVs over time, take a look at this brilliant Wikipedia article:
http://en.wikipedia.org/wiki/Electoral_votes#Chronological_table
