The problem for Democrats, though, is that they might not retake the House even if they managed to perform as well as the Democrats of 2006 or 2008, or the Republicans of 2010.

Why? To extend the wave analogy: The Republican House is on very high ground, and it will take a particularly large wave to bring it down.

Perhaps the easiest comparison is with 2006, when the Democrats gained 31 seats. That number is a convenient example, because the Democrats need to pick up 30 seats this year. Now consider the various ways that the Democrats will be fighting on harder terrain than the Democrats of 2006.

Heading into the 2006 election, the Republicans held 21 seats that were more Democratic than the country as a whole, as measured by the Cook partisan voting index (P.V.I.) — an average of the vote in the last two presidential elections compared with the country. But in 2014, the Republicans won just 10 seats with a Democratic-leaning Cook P.V.I. There are far fewer easy pickings.

How much does this hurt this year’s Democrats? Imagine, for instance, that a Republican sitting in a slightly Republican-leaning district in 2016 is exactly as likely to lose as the average Republican or Democrat in similar positions in 2006 or 2010. Over all, the Democrats would gain around 26 or 27 seats, making Democrats pretty clear underdogs to retake the House.