FRANKFURT — The European Central Bank on Thursday took its most ambitious step yet toward easing the euro zone crisis, throwing its unlimited financial clout behind an effort to protect Spain and Italy from financial collapse.

Mario Draghi, the president of the central bank, won nearly unanimous support from the bank’s board to buy vast amounts of government bonds, a move that would relieve investor pressure on troubled countries but also effectively spread responsibility for repaying national debts to the euro zone countries as a group.

The decision propels political leaders farther down the uncertain and winding road toward a Europe with centralized control over government spending and economic policy, instead of a collection of nation states that sometimes seem to share little more than a currency and a slumping regional economy.

Mr. Draghi demonstrated once again that he may be Europe’s most powerful leader, perhaps the only one capable of brokering an accord among politicians whose national concerns and mistrust of one another have allowed the crisis to boil for two and a half years.