‘This is an assault on democracy.’ D.C. gears up for a wild week.

In Washington, 2021 is beginning with a wild and likely consequential week that could establish which party controls the Senate – and show whether the GOP is splintering into competing factions as President Donald Trump’s term in office nears its end.

In a larger sense, the historic events of these days of transition may define the influence and limits of what Harvard history professor Alex Keyssar calls the Trump school of politics, which combines hard-nosed realpolitik with a disrespect for norms and a focus on stark self-interest.

These factors were...