Barack Obama takes the stage Thursday night for the speech of his career after getting a big boost and a big challenge from his former rival, Hillary Clinton, former President Bill Clinton and Obama’s running mate, Joseph Biden. Senator Clinton’s address, perhaps the best of her career, provided the long-awaited call for her supporters to back Senator Obama. It did something more: It offered the rousing case for the Democratic Party’s core values and strengths that had been largely missing from the convention.

Obama needs to be just as clear about what he stands for, and about why Americans should trust him. On Tuesday night, Clinton passionately argued that the Democratic Party believes in health care for all, progressive taxation, Social Security, fighting against poverty and for gay rights.

Obama got to Denver in large measure on his ability to inspire Democratic voters. He has a strong case to make now against the Republicans’ claim to be the party of prosperity at home and strength abroad. After eight years of President Bush, the country is neither prosperous at home nor respected abroad. But it is not enough to declare Bush’s terms a disaster. Obama’s task is to make the case unequivocally that his ideas and his party’s ideas are the best way to recover from that disaster.