Washington (CNN) For Democrats, this year's governors' races present a once-in-a-decade opening.

This year features 36 gubernatorial contests -- including 23 in Republican-held states -- starting in less than a month with the primary in Illinois to determine who will take on GOP Gov. Bruce Rauner, perhaps the nation's most vulnerable incumbent.

It's the opposite of this year's Senate map, where Democrats are defending 10 seats in states President Donald Trump won in 2016.

What's at stake in many of these races is an opportunity to have a direct hand in the congressional map-making process after the 2020 Census -- something Republicans dominated after the wave election of 2010 swept them into power in statehouse across the country.

Wins in gubernatorial races are perhaps the best way for Democrats to lock in any of the gains they're expected to make in suburban House races in 2018 and potentially 2020.

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