Monday's euro-zone downgrade warning by Standard & Poor's Ratings Services is a muscular move by a firm that was pilloried for its supposed laxity in the financial crisis.

S&P put the long-term sovereign-debt ratings of 15 euro-zone nations, including struggling Italy and Spain, on negative watch. That typically means there is at least a 50% chance of a downgrade within 90 days, but the firm said Monday that it expected to announce any rating changes "as soon as possible" following this week's European Union summit, where...