In a special preview for Axios readers, here's a new analysis by David Wasserman of Cook Political Report, unpacking the GOP's daunting math:

"With 102 days to go, Democrats remain substantial favorites for House control. A big reason: Republicans are defending 42 open or vacant seats, a record since at least 1930." (After the March victory by Democrat Conor Lamb in Pennsylvania, Democrats need to flip 23 seats to take the House).

• "Of Republicans' 42 incumbent-less seats, eight are in districts that voted for Hillary Clinton in 2016, and an additional 13 are in districts where President Trump received less than 55 percent."
• Killer stat: "[S]ince 1992, in situations when a president's party was stuck defending an open seat two years after the president failed to carry it, that party has batted zero for 23 keeping it in their column."

"Fundraising deficits are a growing GOP problem":

• "[I]n 20 of the 42 seats, the leading Democrat raised more than the leading Republican between April and June, including in seven of eight Clinton-carried districts (Rep. Dave Reichert's open WA-08 was the only exception) and 13 of 34 Trump-carried seats."

David's ratings for the 42 Republican open seats: 3 Likely D ... 5 Lean D ... 4 Tossup ... 7 Lean R ... 4 Likely R ... 19 Solid R.

Go deeper:

1. Get more stories like this by signing up for our daily morning newsletter, Axios AM. 
2. Read the full analysis at Cook Political Report.
3. Democratic strength in midterm House races.
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