Decisive moves by the head of the European Central Bank to preserve the euro zone pushed the benchmark American stock index to a four-year high and fueled hopes that the rally would have staying power.

The markets have greeted several previous efforts to solve Europe’s economic woes with euphoria, only to quickly deflate. While there could be some setbacks along the way this time, too, investors suggested that the enthusiasm may not be fleeting. They showed a willingness to dive back into stocks and risky Spanish and Italian bonds and sold safer assets like Treasury bonds. The Standard & Poor’s 500-stock index surged nearly 2 percent, surpassing the peak reached earlier this year and hitting a level last seen in January 2008, before the financial crisis. The Nasdaq composite index rose to its highest point since 2000.

Stocks in the United States were also helped by promising data about the American unemployment picture ahead of Friday’s highly anticipated jobs report.

A weak employment number could easily derail investor optimism. But on Thursday, investors were captivated by the announcement by the president of the European Central Bank, Mario Draghi, that he was ready to start a bond-buying program that would provide what he said was a “fully effective backstop” for the struggling euro.