The Democratic infighting threatening to sink President Joe Biden’s economic agenda is a byproduct of paper-thin majorities just large enough to control Congress but too small to govern.

Liberal and centrist Democrats are at odds over the size and scope of a $3.5 trillion “reconciliation” spending package chock full of the party’s domestic priorities. These differences are typical. But with a 50-seat Senate “majority” that rests on Vice President Kamala Harris’s tiebreaking vote and a narrow, five-seat majority in the House, both factions have the numbers to block the legislation, turning standard haggling into a brutal standoff.