Democratic presidential nominee Hillary Clinton, with her vice-presidential running mate, Tim Kaine, in Miami on July 23. (Patrick T. Fallon/Bloomberg)

A CHASM has opened in this presidential campaign, within parties as well as between them. One aspect of the divide is whether the nation is in a terrible, nearly hopeless fix — or whether it has serious problems but also great, abiding strengths. That division has been much remarked upon since Donald Trump’s dark acceptance speech. But the chasm also concerns whether our politics is capable of addressing the problems. Is it a rigged game in which most players are dishonest and out for themselves or for hidden special interests? Or, is it a flawed but essentially noble enterprise in which people choose representatives who strive to improve the public welfare? When she accepts the presidential nomination Thursday night, Hillary Clinton will position herself on one side of this divide or the other.

Her convention already has furnished an eloquent champion of the more optimistic vision: first lady Michelle Obama, the star of the Philadelphia show so far. Ms. Obama electrified the convention Monday night with a much-needed schooling on what America is about. She used — and embodied — words such as “decency” and “grace” and “character” and “convictions.” She talked about the need to leave something better for the next generation. She drew contrasts without engaging in schoolyard taunts.

Ms. Clinton’s choice for vice president, Virginia Sen. Tim Kaine, also personifies the positive side of politics. At this convention he has been derided by some supporters of Sen. Bernie Sanders (I-Vt.), who mistake Mr. Kaine for a centrist. What confuses them is Mr. Kaine’s reluctance to disparage opponents and his willingness to work across the aisle. But he has done so, consistently, in the service of liberal ideals. As a civil rights lawyer, city councilman, mayor, governor and senator, Mr. Kaine has put his faith — and hard work — in the system.

Mr. Trump offers the greatest contrast to these values, both in what he says and how he behaves. But on the Democratic side, too, there is dissent. When Sen. Elizabeth Warren (D-Mass.) tells the delegates that the system is, indeed, rigged, some listeners will conclude that they might as well vote for the candidate who promises nihilistically to blow it all up — not the nominee who touts herself as an agent of change because for 40 years she has worked doggedly within that system for incremental improvement.

Ms. Clinton cannot minimize the country’s challenges or its citizens’ anxieties. But Ms. Obama and Mr. Kaine understand that disparaging the system feeds hopelessness and cynicism — and puts you on the wrong side of American history. “Don’t let anyone ever tell you that this country isn’t great, that somehow we need to make it great again,” Ms. Obama said. “Because this, right now, is the greatest country on Earth.” For his part, Mr. Kaine on Saturday borrowed the words of one of his favorite presidents, Harry Truman: “America was not built on fear. America was built on courage, on imagination and an unbeatable determination to do the job at hand.”

As the first woman to capture the presidential nomination of a major party, Ms. Clinton will help illustrate the truth of those words Thursday night. In her own words, she will have to convince a nation that she can help keep that creed valid in the years ahead.