(CNN) Midterm elections are about one thing: Base enthusiasm.

Unlike presidential elections -- where casual voters dial in and turn out -- midterm elections, historically speaking, tend to be the dominion of only the most committed party warriors. And those most committed party warriors almost exclusively hail from the extreme edges of both the Democratic and Republican coalitions -- i.e. the most liberal and the most conservative elements.

Which brings me to the chart above. It's from a brand new national Pew poll and shows one thing very clearly: The Democratic base is more passionate than its Republican counterpart about the 2018 midterms.

Almost 7 in 10 (69%) of Democrats say they are "looking forward" to the 2018 midterms, as compared to 58% of Republicans. Dig deeper and the enthusiasm gap widens. More than 8 in 10 self-described "liberals" are looking forward to the coming election, as compared to 61% of "conservatives."

A look back at the 2010 and 2014 midterms reveals why these numbers matter so much.

Read More