In 20 years of covering presidential politics, I never saw a run of buoyant campaign rallies, boffo fund-raising, ecstatic social media and rank-and-file rapture for a candidate like Kamala Harris had in July and August, capped off by her Democratic convention speech. The next day, I sounded a note of caution about how joy is not a strategy, an argument that some readers disagreed with and some colleagues saw differently.

But a week after her speech, on Aug. 29, Harris faced her first real test — and, I’ve come to conclude, she fumbled it badly.

Her appearance that night on CNN — her first and only major interview since President Biden dropped out — was a missed opportunity: Harris gave canned or vague answers about why she had changed big positions from her 2020 campaign, she didn’t explain persuasively how she would lower the cost of living, and she responded blandly about what she would do on Day 1 as president. But most of all, I think, she didn’t leave strong positive impressions on undecided voters and lukewarm independents and Democrats.

The CNN interview looms large for me in part because Harris is running against a man who is unfit for office, who did enormous damage to the nation as president, who frequently veers into incoherence and lies in his news conferences and interviews — and yet who, for all that, is tied with Harris in leading polls.

Something seems to have happened in the past couple of weeks. As my newsroom colleague Nate Cohn wrote on Sunday: “Is Kamala Harris’s surge beginning to ebb?” CNN was one interview, not a trend, but I think it was revealing that a joy-driven campaign takes you only so far.

I checked in recently with our Times Opinion panel of young, undecided voters, which we are tracking through Election Day. Most in this group of 14 voters praised Harris’s convention speech; five said it made them more likely to vote for her. By contrast, seven said her answers on CNN made them less likely to vote for her.

“Harris did a poor job of explaining how she would ease inflation,” said Laura, a 20-year-old legal intern in Maryland, who was one of those less likely to support Harris after the interview. Lillian, a 27-year-old Virginian who gave Harris strong marks for her convention speech, could not come up with one positive takeaway from the interview.

For most in our group, Tuesday’s debate is critical in choosing a candidate. They want a better handle on who she is; they want stronger answers that build trust in her.

Big tent-pole events have an outsize impact in a tight race. The debate is one of Harris’s best chances to win over the doubters and undecideds and energize her momentum against Trump. No debate has ever felt more important.