Full Democratic control of the levers of federal lawmaking under President Joe Biden rests on the slimmest-possible advantage in the U.S. Senate, where any party-line vote will result in a 50-50 tie. The even Senate split, rare in American history, gives the Biden administration little margin for error and is subject to a power-sharing agreement between the Democratic and Republican leaders.

The last time Senate Democrats and Republicans were split 50-50 was two decades ago. Coming to an agreement was a thorny process then, and is more difficult now, with nerves made raw by the fatal Jan. 6 Capitol riot ...