Citigroup, whose shares dropped more than 25% on Thursday, has asked the Securities and Exchange Commission to once again ban short-selling of financial stocks, a source said. The Financial Services Roundtable is also urging regulators to once again prohibit the investment strategy. Scott Talbott, senior vice president of government affairs for the Roundtable, said officials have been discussing the matter with regulators and others since the ban was lifted. "When conditions warrant, you want to prevent a downward spiral for shares. Investors are acting on panic now. ... We want markets to operate efficiently," Mr. Talbott said.