Ryan Teague Beckwith had this information in Bloomberg. Here is an excerpt:

Democratic and Republican state legislatures have put the competitive congressional district on the endangered species list, bolstering the GOP advantage in this year’s midterm elections but also setting up an even more polarized Congress next year.

There are just 13 toss-up seats out of the 269 districts in redistricting completed so far, according to ratings from the non-partisan Cook Political Report. The U.S. has 435 congressional districts and not all state maps are yet completed, with some facing legal challenges.

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