48 vulnerable seats?

The non-partisan Cook Political Report believes that 48 GOP-controlled House seats are in play. This is up from 18 in January. However, on why this is less important than it seems, let me repeat the most important quotation in the Los Angeles Times' coverage:
"It's one thing to be close," said Scott Lasley, a political scientist at Western Kentucky University, who is tracking the contest between Lewis and Democrat Mike Weaver. "It's another to defeat an incumbent."
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