With the current Senate evenly divided and Democrats having the narrowest of majorities only thanks to Vice President Kamala Harris's tie-breaking vote, every seat will literally matter for both parties in next year's midterm elections.

And while the map slightly favors Democrats by the numbers and it's Republican open seats that have put them even more on the defense in several key states, Republicans are, understandably, optimistic about history being on their side for President Joe Biden's first midterm election and hoping that voters will be ready to end unified Democratic control of Washington.

However, in such a scenario, Senate losses have been much more marginal than shifts have been in the House. That's largely because unlike the House, where all 435 members are up for re-election (and this cycle of course must contend with redistricting too), which third of the Senate is on the ballot deeply matters. For example, in 2018, even as Democrats flipped 40 seats to win back control of the House as a backlash to President Donald Trump, Republicans managed to net two seats

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