Sources say Rep. Jerry Nadler of New York and Sen. Ed Markey of Massachusetts are the lead authors of the bill.
New York Rep. Mondaire Jones, a co-sponsor of the bill, tweeted Wednesday evening, "Supreme Court expansion is infrastructure."
Supreme Court expansion is infrastructure.— Mondaire Jones (@MondaireJones) April 15, 2021
The news comes days after President Joe Biden announced a commission to study a possible expansion of the court.
Under former President Donald Trump, the court swung decisively toward Republicans after he appointed Amy Coney Barrett, Neil Gorsuch and Brett Kavanaugh.
Currently, Republicans hold six of the seats, and Democrats have three.
The Supreme Court hasn't always consisted of nine justices. The number has changed six times before settling at the current number in 1869.