Unlike in 2020, Georgia’s U.S. Senate runoff won’t decide whether Chuck Schumer or Mitch McConnell is majority leader. But it will determine whether Democrats have a true majority. If they do, Vice President Kamala Harris, who’s cast 26 tie-breaking votes, will have less to do—but that’s the least important implication.

With an evenly divided Senate, any single Democrat can prevent legislation from passing without Republican support. Even when the holdout eventually gives in, the result may be less extreme, as when West Virginia’s Joe Manchin brought the cost of the misnamed Inflation Reduction Act down to down to $750 billion, plus another $1.2 trillion for a separate infrastructure piece, from its initial $6 billion.

A 50-50 Senate also means a single Democrat can prevent a party-line confirmation. Mr. Manchin forced Neera Tanden’s withdrawal as director of the Office of Management and Budget and Sarah Bloom Raskin as a Federal Reserve governor by announcing he’d vote against them. David Weil, the radical academic who would have headed the Department of Labor’s Wage and Hour Division, went down to defeat when Arizona’s Kyrsten Sinema and two other Democrats voted against him.

An even split makes every Democratic senator the deciding vote on every party-line measure that requires a simple majority. That helps provide clarity at re-election time. With 51-49, Mr. Schumer could let two at-risk senators be absent when the chamber votes on bills or confirmations that would be particularly unpopular back home.