WASHINGTON - The chief of JPMorgan and a contingent of Wall Street regulators came under aggressive questioning on Tuesday by members of Congress over the banking giant’s investment losses and the landmark law aimed at expanding oversight of the nation’s financial industry.

Massachusetts Representative Barney Frank, the ranking member of the Financial Services Committee, led Democrats in a spirited defense of the Wall Street overhaul law that bears his name, suggesting that stricter oversight and tougher rules could protect against a repeat of JPMorgan’s May announcement it had lost $2 billion.