In their seventh debate, this time in Flint, Mich., Hillary Clinton and Bernie Sanders agreed on the root causes of that city's drinking water crisis. They both called for a massive federal intervention and investigation of the lead poisoning there and urged that the state's Republican governor, Rick Snyder, either resign or be recalled.

But the two Democratic candidates also clashed over the role of trade deals in the deterioration of Michigan's economy, the usefulness of the Export-Import Bank and the state of manufacturing in America generally.

They also disagreed about gun control and the best way to expand health care coverage to the maximum of Americans. They had predictably different views of whether President Bill Clinton helped or hurt African-Americans during his eight years in office.

Sometimes, the contrast between their positions, aims and personalities created visible tension. When Clinton talked about cracking down on U.S. companies that move jobs overseas or move their corporate headquarters to duck U.S. taxes, Sanders was contemptuous.

"I am very glad," Sanders said, "that Secretary Clinton discovered religion on this issue. But it's a little bit too late."

At another juncture, Clinton noted Sanders' vote against the 2009 bailout for the auto industry as a contrast between them.

"If you are talking about the Wall Street bailout where some of your friends destroyed this economy," Sanders began. Clinton began to interrupt, then Sanders said: "Excuse me, I'm talking."

"If you're going to talk, tell the whole story," said Clinton.

"Let me tell my story, you tell yours," he responded.

There were also hostile exchanges on the issue Clinton's team regards as Sanders' greatest vulnerability in the primaries. Clinton noted that Sanders voted against a bill making gun manufacturers liable for crimes committed with their products, calling it a blanket immunity enjoyed by no other industry.

Sanders said Clinton was talking about a position whereby no guns could be manufactured in the U.S. at all, adding that he disagreed with that.

Both candidates admitted they did not and could not really know how it felt to be a person of color in the U.S. Each related experiences in their youth that brought them into confrontation with the racial schism of their time. Clinton spoke of exchanges between her suburban church and the youth of inner-city churches in Chicago. Sanders related his own arrest for protesting discrimination in housing in the same city, when he was a student at the University of Chicago.

But despite the frequent disagreements and reliance on sarcasm, the debate between these two candidates was a broadly collegial affair when compared with the two most recent Republican debates.

"I just want to make one point," said Clinton near the end of the evening. "We have our differences and we get into vigorous debate about issues. But compare the substance of this debate with what you saw on the Republican stage last week."

Sanders agreed, saying either he or Clinton would invest "a lot of money into mental health, and when you watch these Republican debates you know why we need to invest."

If you have watched all six of the previous debates among the Democratic candidates, you know well the arc of the contest thus far.

Initially, the Democratic National Committee wanted only four debates, and scheduled this small number in time slots all but guaranteed to minimize the audience watching in real time. Pressure to expose the candidates and create more competition between them led to additional debates.

The stage at first included not only Hillary Clinton and Bernie Sanders but two other contestants as well: former Sen. Jim Webb of Virginia and former Gov. Martin O'Malley of Maryland.

Webb dropped early, complaining about his scant speaking time. O'Malley lasted until the Iowa caucuses, where his minor fraction of the vote indicated he was not connecting with voters.

Since then, Sanders and Clinton have met several times, and their encounters have been progressively less cordial. Sanders had been generous toward the former secretary of state in the first debates, even helping her deflect criticism for keeping official business on her private server while in office.

But as the Sanders campaign matured into a major competitor for Clinton, Sanders became more challenging, even hostile. In February in Milwaukee, he interjected a quick jab: "You're not the president yet."

The Flint debate featured several similar moments when, without becoming overtly nasty, Sanders managed to assert his presence and challenge the air of dominance Clinton has sought to project.

On this occasion, Sanders could also note, as he did, that he had won three of the four states where voting took place over the weekend: Kansas, Nebraska and Maine. Big margins in these states had given him a slight edge over Clinton in the delegate allocations for the weekend, despite her overwhelming win in Louisiana, a primary state where far more votes were cast than in the three caucus states combined.

Clinton could say she won more votes over the weekend than any of her rivals in either party, and she could say she'd won a million more votes than Donald Trump thus far in all the primaries and caucuses around the country.

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