[Speakers, start time, schedule and more: Here’s how to watch the Democratic National Convention.]The first-ever virtual convention begins tonight, as Democrats gather — sort of — to nominate Joseph R. Biden Jr. for the presidency and Senator Kamala Harris of California as his running mate. Even with coronavirus-imposed restrictions, the speeches over the next few days may be the opposition party’s best chance to break through a seemingly endless wall of chaotic breaking news and deliver a memorable message to the country about why voters should vote President Trump out of office and give Democrats a chance to govern.That is the goal, anyway. Even people deeply involved in organizing the convention acknowledge that it is an ungainly experiment, with two hours of live programming each night, speakers beamed in from different cities one by one and no live audiences to laugh, cheer, clap or boo.The climactic events of the week come on Wednesday and Thursday, when first Ms. Harris and then Mr. Biden, the former vice president, are set to deliver their acceptance speeches. But the party is hoping to hook voters’ attention on Monday, with speakers meant to highlight the diversity of Mr. Biden’s political coalition in terms of ideology, geography, race and more.Here is some of what we’ll be watching for.What will Democrats promise the country?It may be the biggest substantive question of the week. We know that the central issues of the campaign are Mr. Trump’s dysfunctional administration and his mismanagement of the coronavirus crisis. And we know that the Biden campaign has issued a sizable array of policy plans, on matters including student debt, climate, child care and elder care. We also know that the big thematic frame of Mr. Biden’s message is that the country’s soul is being put to the test under Mr. Trump and the 2020 election is a chance to redeem it.What we have not yet heard from Mr. Biden is the kind of read-my-lips statement of purpose by which his administration might be judged. Think of Mr. Trump’s pledge during the 2016 presidential campaign — often cited now by his critics — that he alone could fix the corruption in Washington and strengthen America’s economic place in the world.We’ll see how clearly Mr. Biden and the rest of his speakers tie together his policy promises with the larger themes of his candidacy, and how richly they convey to Americans the ways their lives might change after four years of a Biden-Harris administration.Is there an anti-Trump consensus, or cacophony?The lineup of speakers on Monday night is eclectic, and deliberately so: With headliners ranging from Senator Bernie Sanders of Vermont on the left to Senator Doug Jones of Alabama in the middle and former Gov. John Kasich of Ohio, a Republican, to the right of center, the idea is to showcase a broad spectrum of Biden backers who are united by their opposition to the current president. Mr. Biden has been seeking to run as a candidate of national unity, and the opening night of his convention will show it.But if the speakers agree on one big thing — the need to defeat Mr. Trump — their differences are also considerable. So, given their disagreements, how much will they talk about a common agenda beyond just ousting Mr. Trump? And will their calls for a new spirit of political cohesion come across as inspiring and soothing, or superficial and overly familiar?Four years ago, Hillary Clinton’s convention tried to convey a similar kind of united front against Mr. Trump, with only limited success. But Mr. Biden has wider political appeal than Mrs. Clinton, and the circumstances of this election are very different.The Obama factorThe most popular figure speaking on Monday night — or at any time this week — will be Michelle Obama. The former first lady is seen by Democrats not only as a symbol of a better time, but as an electric speaker and a champion of the causes they care about the most. She can speak to Mr. Biden’s character and his service as vice president like no one else, save Jill Biden and Barack Obama. And because of her own pathbreaking identity, she could play a special role in introducing Ms. Harris to those Americans who still do not know much about her.Mrs. Obama has also put voting rights and voter participation at the top of her political agenda since leaving the White House. Those subjects are particularly urgent now, amid widespread fear that the pandemic will make voting far more difficult and growing alarm that the Trump administration is undermining the Postal Service to thwart vote-by-mail efforts at the state level. Should Mrs. Obama take up the issue herself, she would add a powerful voice to the debate.Sanders and the leftThe last time Mr. Sanders addressed a Democratic convention, it was to help nominate a candidate, Mrs. Clinton, with whom he shared a frigid relationship. His supporters were demoralized and newly angered by hacked documents that seemed to confirm their suspicions that the Democratic National Committee had aligned against Mr. Sanders in the 2016 primary campaign. While he gave a firm endorsement to Mrs. Clinton, it did not assuage his supporters’ grievances.Mr. Sanders is appearing now in a different context. He ended his battle with Mr. Biden months ago, and the two men have enjoyed a warm political relationship. But there are still serious reservations about Mr. Biden among the most liberal voters, many of whom feel emboldened by their victories further down the ballot during the primary season. Mr. Sanders is in a unique position to speak to these voters and explain what he believes they can accomplish in a Biden presidency. Will he do that, or focus on promoting the core of his own agenda?The governors speakIn a convention reshaped by the pandemic, no voices may speak more directly to the public health crisis than those of the governors on the front lines. State leaders will be speaking throughout the week, but the two openers on Monday are big ones: Govs. Andrew M. Cuomo of New York and Gretchen Whitmer of Michigan. They were two of the most prominent figures battling the initial wave of outbreaks in March and April, soaring in personal popularity and, in Ms. Whitmer’s case, earning a place on Mr. Biden’s vice-presidential short list.Both governors are positioned to give detailed testimonials about the ravages of the virus and what the country will need to recover. Both Mr. Cuomo and Ms. Whitmer are seen as potential — even likely — future presidential candidates. Up to this point, Mr. Cuomo has tried to avoid antagonizing Mr. Trump, arguing that he needed to preserve a working relationship with the president on matters of public health. Will that change in a partisan convention?