During a week in which Democrats formally finalized their takeover of the US Senate's instruments of power, and flexed their legislative muscles by pushing through a party-line vote to pave the way for a massive spending bill, it seems like an opportune moment to reflect on how we got here. Republican voters largely held the line, despite a daunting Senate map, in November. After election night, it very much looked like Democrats would net just one single seat in the upper chamber, allowing Mitch McConnell to remain majority leader, and force the Biden administration into tough negotiations on a wide...