A blue wave could happen and still hit a wall

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(CNN)As we get closer to November, I gain confidence that an electoral "wave" will develop the likes of 1946, 1994, 2006 and 2010.

Unlike the elections in those years, however, I'm not sure a wave will necessarily mean the minority party will wrestle away control of the House.
How is this possible?
It gets back to a question the New York Times' Nate Cohn and the Cook Political Report's Amy Walter asked: what is a wave? I'd argue a wave doesn't just need to be measured by seats won. It can be measured by votes won. It's on this score that Democrats are in a very strong position historically speaking.
    In an average of live interview polls conducted over the last month, Democrats (the minority party) held a 7.5-percentage point lead on the generic congressional ballot. No minority party since World War II has won the national House vote in a midterm election by more than the 8.5 percentage points Republicans won it by in 1946.
    The next highest was the eight-point margin Democrats took the national House vote by in 2006. The current polling for the upcoming election is between 2006 and the seven-point margin the minority Republicans won it by in 1994 and 2010. That is, Democrats are doing really well for a minority party.
    The problem for Democrats isn't lack of popular support. It's how that support gets translated into seats. A 7.5-point win in the national House vote puts Democrats right in the area of where they need to be for a net gain of 23 seats to win a majority of seats. The level of disparity between seats and votes won is far from exact, though it illustrates a rather important point that I've spoken about at length.
    It's not unusual historically speaking for the minority party to need more than a majority of votes (cast for the two major parties) to win a majority of seats. That's because incumbents tend to outperform the national environment, and the majority party usually has more incumbents running.
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    What is unusual about 2018 is how great the disparity is between votes and seats is likely to be because of geographic sorting and gerrymandering. Usually, the minority party would need to win the House popular vote by a few points to win control, not seven.
    It's this historic difference that makes it difficult, in my mind, to judge a wave on the sole basis of seats won and lost.
    Now obviously, in terms of control of the government, it's seats that matter. It's also arguable that seats should be included in anyone's definition of a wave.
    It just strikes me as unreasonable to expect that Democrats to win a net gain of much more than 23 seats given the vote/seat disparity and that no minority party in a midterm in the modern era has done better than an 8.5-point popular vote win. If Democrats are able to do that, it would be extraordinary.