Five U.S. agencies will finish the Volcker rule tomorrow after more than three years of Wall Street resistance to its limits on trading and investing. Lawmakers and their allies who want to rein in big banks are ready to pounce if it isn't strict enough.

Politicians and advocates—some Democrats, some Republicans—who blame the 2008 financial crisis on deregulation express concern that the Volcker rule won't adequately block banks from making risky bets with their own money. If the rule is too weak, they say, it will add fuel to a push to reinstate a Depression-era law known as Glass-Steagall that split banks and securities firms until 1999.

Such vows suggest U.S. lenders planning to challenge the ban in court risk a political backlash. A 2011 draft of the rule, required by the Dodd-Frank Act at the urging of former Federal Reserve chairman Paul Volcker, disappointed some politicians and organizations that wanted a stronger ban. Lawmakers already have drafted legislation.