FLINT, Mich. -- The campaign for the Democratic presidential nomination is supposedly over, with former Secretary of State Hillary Clinton clearly ahead of Vermont Sen. Bernie Sanders and on her way to capturing a majority of delegates.

But for two hours on Sunday night, Clinton and Sanders debated as if the outcome was very much in doubt -- sparring over everything from trade and corporate welfare to health care and guns.

The debate, held in Flint, Michigan, was feisty and even cantankerous. Clinton and Sanders had several exchanges where they tried to talk over one another; Sanders, as always, brought his full array of facial expressions.

But the debate was also deeply substantive -- an argument between two seasoned politicians who have obviously given a lot of thought to the problems facing America, and what they would do about them.

The contrast to the Republican debate from last week, with its juvenile insults, could not have been more stark. Instead of discussing penis length, Clinton and Sanders argued over the sizes of their respective infrastructure programs.

In short, both Clinton and Sanders looked ready for prime time.

The debate began, appropriately, with an extended discussion of Flints water crisis -- and what the candidates would do, both to help the people of Flint who suffered because of lead in the water and to prevent future tragedies. And it was one of the few discussions of the night when Clinton and Sanders were more or less saying the same thing. Both called for setting up programs for Flint families, both demanded the resignation of Michigan Gov. Rick Snyder (R), and both called for testing of lead in pipes across the country.

Sanders said that it is beyond belief that children in Flint, Michigan in the United States of America in the year 2016 are being poisoned. That is clearly not what this country should be about.

Clinton said, This is not the only place where this kind of action is needed. We have a lot of communities right now in our country where the level of toxins in the water, including lead, are way above what anybody should tolerate. I want to tackle this problem across the board.

But then an audience member from Flint asked what the candidates would do to keep manufacturing jobs from leaving the country -- and the sparks started flying. Clinton said she wanted to use a mix of carrots and sticks, by which she meant new incentives to create factories in the U.S. and a proposal (just announced a few days ago) to claw back tax breaks from companies that transfer operations overseas.

Sanders used the occasion to pounce -- attacking Clinton, as he has before, for backing the North American Free Trade Agreement and other trade deals. I am very glad that Secretary Clinton discovered religion on this issue, but it's a little bit too late, Sanders said.

Secretary Clinton supported virtually every one of the disastrous trade agreements written by corporate America.