For more than a year, the message from President Biden was dark. The very foundations of the country were at stake in the election, he said. An American apocalypse was on the horizon if former President Donald J. Trump was re-elected.

With Mr. Biden out, and Vice President Kamala Harris now at the top of the Democratic ticket, the tone of the campaign has changed. But even amid the more joyful, optimistic vibe, protecting democracy has been front and center at the party convention this week.

Speakers onstage have sought to navigate those seemingly diverging sentiments of hope and fear, expressing optimism about the November election but also invoking attempts by Mr. Trump and his supporters to overturn the 2020 election and continuing challenges to voting rights across the country.

“Welcome to the democracy convention,” Representative Jamie Raskin of Maryland bellowed from the podium on Monday night. “Make no mistake, a man who uses fraud, theft and violence to take power will commit any crime to keep it.”

Settling on a messaging strategy will be critical for Democrats, as the issue of democracy has quickly become one of the most pivotal in U.S. elections. A Fox News poll in June found that 88 percent of voters thought the future of American democracy was either extremely important or very important to how they would vote in November. But voters differed in how they defined their concerns. Some expressed worry about free and fair elections; others focused on defending personal rights and freedoms.

At the polls, the issue has seemed to favor Democrats. In the 2022 midterm elections, Mr. Biden led a focused attack on political extremists seeking to disrupt the democratic process, pairing the issue with abortion rights and leading to Democratic wins in several critical states. Images of the Jan. 6, 2021, riot at the Capitol have been fodder for attack ads produced for Democratic candidates up and down the ballot.