The Democrats have completed their convention, and I'm more optimistic than I was a week ago that Barack Obama can be elected president in November. That'll happen, however, only if voters look for substance and breadth, and if they refused to be swayed by lies and attack ads.

Last night's speakers, like those of the previous nights, laid out the stark differences between Obama and John McCain.

Predictably, Al Gore gave a passionate lecture on global warming.

"We are facing a planetary emergency which, if not solved, would exceed anything we've ever experienced in the history of humankind," Gore said. "There are times in the history of our nation when our very way of life depends upon awakening to the challenge of a present danger, shaking off complacency to rise, clear-eyed and alert, to the necessity of embracing change." One of those time, said Gore, is now.

From Republican Susan Eisenhower, granddaughter of the late president: "Too many people in power have failed us. Belligerence has been a substitute for strength; stubbornness for leadership; and impulsive action has replaced measured and thoughtful response."

Obama "has the energy and the temperament to lead this country," Eisenhower said. "He knows that we can either advance on the distant hills of hope or retreat to the garrisons of fear.... Unless we squarely face our challenges, as Americans together, we risk losing the priceless heritage bestowed on us by the sweat and the sacrifice of our forbearers. If we do not pull together, we could lose the America that has been an inspiration to the world."

In his 43-minute acceptance speech, Obama tried to combine campaign combativeness with an inspirational vision.

He repeatedly attacked both McCain and the Bush administration: on the economy, on foreign policy, on the treatment of veterans, on health care. And he went after McCain's attempt to paint him as a celebrity.

"I don't know what kind of lives John McCain thinks that celebrities lead," he said, "but this has been mine. These are my heroes," and he spoke of a grandfather who fought in Patton's Army, a mother who raised him while going to school and working, a grandmother who sacrificed for him, the laid-off workers he met during in South Side Chicago."

He attacked the McCain campaign's tactics. "The times are too serious, the stakes are too high for this same partisan playbook," he said. "So let us agree that patriotism has no party. I love this country, and so do you, and so does John McCain. The men and women who serve in our battlefields may be Democrats and Republicans and independents, but they have fought together and bled together and some died together under the same proud flag. They have not served a Red America or a Blue America - they have served the United States of America. So I've got news for you, John McCain. We all put our country first."

And during a discussion of foreign policy, he underscored one of the most important differences between himself and John McCain: McCain's belligerence. "Just as we keep our promise to the next generation here at home, so must we keep America's promise abroad," he said. "If John McCain wants to have a debate about who has the temperament, and judgment, to serve as the next Commander-in-Chief, that's a debate I'm ready to have."

In the past, Obama has been criticized for not being specific about what he would do as president. Last night's speech had lots of specifics: tax cuts; investment in renewable energy, nuclear power, and clean-coal technology; investment in education; a promise to lower the cost of health insurance. The price will be enormous, and Obama offered few specifics on how he hopes to pay for his initiatives other than "closing corporate tax loopholes and tax havens" and cutting fat from the federal budget.

But mostly, this was a speech spelling out his philosophy - letting the public see how he thinks, the mindset he'll use as he tackles the enormous challenges the country faces. Obama, as a recent article in the New York Times Magazine put it, "is both more left-wing and more right-wing than many people realize." He emphasizes the importance of market forces more than many liberal Democrats of the past have. He emphasizes the need for both government and individual responsibility.

The nation's promise, he said last night, is "a promise that says the market should reward drive and innovation and generate growth, but that businesses should live up to their responsibilities to create American jobs, look out for American workers, and play by the rules of the road.

"Ours is a promise that says government cannot solve all our problems, but what it should do is that which we cannot do for ourselves - protect us from harm and provide every child a decent education; keep our water clean and our toys safe; invest in new schools and new roads and new science and technology."

"Our government should work for us, not against us," he said. "It should help us, not hurt us. It should ensure opportunity not just for those with the most money and influence, but for every American who's willing to work."

"That's the promise of America," he said, "the idea that we are responsible for ourselves, but that we also rise or fall as one nation; the fundamental belief that I am my brother's keeper; I am my sister's keeper."

"Yes, government must lead on energy independence," he said, "but each of us must do our part to make our homes and businesses more efficient. Yes, we must provide more ladders to success for young men who fall into lives of crime and despair. But we must also admit that programs alone can't replace parents; that government can't turn off the television and make a child do her homework; that fathers must take more responsibility for providing the love and guidance their children need."

The speech was long, and some of it was so predictable that you just wanted it to be over. But sprinkled through it was the eloquence and vision that has inspired his followers. "We Democrats have a very different measure of what constitutes progress in this country," he said. "We measure progress by how many people can find a job that pays the mortgage; whether you can put a little extra money away at the end of each month so you can someday watch your child receive her college diploma. We measure progress in the 23 million new jobs that were created when Bill Clinton was president - when the average American family saw its income go up $7,500 instead of down $2,000 like it has under George Bush.

"We measure the strength of our economy not by the number of billionaires we have or the profits of the Fortune 500, but by whether someone with a good idea can take a risk and start a new business, or whether the waitress who lives on tips can take a day off to look after a sick kid without losing her job - an economy that honors the dignity of work."

America has lost "our sense of common purpose - our sense of higher purpose," he said. "And that's what we have to restore.

"We may not agree on abortion, but surely we can agree on reducing the number of unwanted pregnancies in this country. The reality of gun ownership may be different for hunters in rural Ohio than for those plagued by gang-violence in Cleveland, but don't tell me we can't uphold the Second Amendment while keeping AK-47s out of the hands of criminals.

"I know there are differences on same-sex marriage, but surely we can agree that our gay and lesbian brothers and sisters deserve to visit the person they love in the hospital and to live lives free of discrimination.

"Passions fly on immigration, but I don't know anyone who benefits when a mother is separated from her infant child or an employer undercuts American wages by hiring illegal workers."

"This too is part of America's promise," Obama said, "the promise of a democracy where we can find the strength and grace to bridge divides and unite in common effort."

"This country of ours has more wealth than any nation, but that's not what makes us rich," he said. "We have the most powerful military on earth, but that's not what makes us strong. Our universities and our culture are the envy of the world, but that's not what keeps the world coming to our shores.

"Instead, it is that American spirit - that American promise -that pushes us forward even when the path is uncertain; that binds us together in spite of our differences; that makes us fix our eye not on what is seen, but what is unseen, that better place around the bend."

How good was Obama's speech? Less inspirational, I thought, than some of his others. But CNN commentator David Gergen calls the Obama speech "a political masterpiece," "a symphony."

That's strong praise, particularly considering the source. Gergen served as an advisor to Presidents Nixon, Ford, and Clinton and was communications director for President Reagan.

More important, of course, will be the public reaction, not only to Obama's speech and to the speeches of the past four days, but also to what candidate Obama says and does in the next two months.

The Democrats' convention was energizing, often inspiring, frequently emotional. The question, though, is whether they were preaching primarily to the choir. The question is how many of the millions who watched the convention on television were voters still trying to make up their minds in this crucial election campaign.