SEATTLE — One district had been a Republican heartland of freight-haulers and timber-workers before a shift toward suburbs and tech. Another was historically Democratic from the power of the railroad unions, then veered toward the Republicans. A third is liberal on its western side, conservative to the east, and locked in a battle over which side will prevail.

Three of Washington State’s 10 congressional districts are in play in Tuesday’s election — all considered competitive by the nonpartisan Cook Political Report — and so all three are potentially crucial to Republicans in holding their majority in the House of Representatives, and to Democrats in their hopes of taking back the House. Two seats are held by Republican incumbents, while the third is an open seat being vacated by a Republican.

Washington State as a whole leans left these days, from the dominance in population of the Seattle area — both senators are Democrats, as are six of the 10 House members. But the state’s three in-play districts do not fit that pattern, or really any pattern. All are starkly different from one another in economics, demographics, history and in the pitches that the candidates are making to voters.

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Here is a look at three key races that could be pivotal on Tuesday — though the results may not be known until the wee hours of Wednesday or later, given that ballots in the state’s vote-by-mail election can be postmarked as late as Election Day itself.