"If you win this race, it'll send ripples throughout the country," said a jacketless Biden in downtown Birmingham's convention center, as Jones looked on. "I want to be clear: Don't do it for that reason. Do it for Alabama."
Democrats, who have not seriously contested a Senate race in Alabama since Bill Clinton was president, see an opportunity now - but are cautious about how to take it. They see how Jones could excite the party's base but worry about him riling up Republican voters. They've watched Alabama's Republicans sour on their party leadership but not yet on President Donald Trump.
The result, one week after far-right former judge Roy Moore secured the Republicans' Senate nomination, is a high-stakes contest in which both national parties are hesitant to fully engage. Jones has begun to attract more interest from national donors, and a representative of the Democratic Senatorial Campaign Committee was on hand at the Biden rally.
But Biden, who has known Jones for years and previously urged him to run for office, is expected to be Democrats' highest-profile surrogate in a state where the national party's brand is almost toxic. Two polls taken since the election, one for BuzzFeed News and one for JMC Analytics, have found Jones trailing Moore by single digits but also found President Trump enjoying majority support from voters.
Republicans, who poured more than $10 million into the state to beat Moore, have congratulated him - but have turned off the spigot, at least for now. "I bet you by the end of the day, I'll have raised more money than Roy Moore," Jones said last week.
Since Moore's upset over Sen. Luther Strange, R-Ala., Jones has raised nearly $1 million - more than any Democratic candidate for the office in 21 years. Democrats are cautiously optimistic that Jones, who has never run for office before, might even outspend Moore, a grass-roots campaigner who has never raised much money for his races.
But a party that has increasingly come to see demography as destiny, and that ran a disastrous 2016 campaign by arguing that Trump would embarrass the country, is wrestling with Alabama. To national donors, Moore's outrages - speculating that the 9/11 terrorist attacks were a result of the United States' sins, for instance, or refusing to recognize same-sex marriages - make him unfit for office.
In Alabama, Democrats are taking advantage of that impression, while impressing a slightly different message on the more-moderate Republican voters who could make the race close. They warn that as a judge twice expelled from Alabama's high court, Moore disregarded "the rule of law," and that Jones never would - a bit of rhetorical jujitsu against Republicans for whom the phrase "law and order" can evoke all sorts of problems with liberalism.
"When you are on the right side of history and the right side of justice, you can do anything," Jones said at the rally. "Roy Moore is not on the right side of any of those things."
Biden amplified that message, adding dramatic details to Jones's work as a U.S. attorney who prosecuted an abortion clinic bomber and - more famously - convicted the Ku Klux Klan members who bombed Birmingham's 16th Street Baptist Church.
"Doug removed 40 years of stain and pain from this state," said Biden. "The Klan needed to know that justice would follow them to the gates of hell."
Deep South Democrats, who were nearly wiped out of power during Barack Obama's presidency, argue that Jones can close the gap with Moore by emphasizing his own store - not by counting on Moore's radical views to sink him. Their best-case scenario resembles what happened in 2015 in Louisiana, when then-retiring senator David Vitter, who had fought through the aftermath of a prostitution scandal, was upset by now-Gov. John Bel Edwards, D-La.
John Arsement, a Louisiana media strategist who helped shape that year's message, warned that Democrats would lose a simple referendum on Moore. "Getting fired for dragging around a Ten Commandments statue is not the same as cheating on your wife with a hooker," he said. Instead, Democrats needed to sell Jones.
"You're not going to win solely on the issues themselves," said Arsement. "The GOP has a pretty good monopoly on these issues. What we did in Louisiana was help voters see John Bel as not just the Democrat in the race; they saw him as the U.S. Military Academy graduate, the Army ranger."
Edwards, however, was personally antiabortion, an issue that helped blur some lines for voters. Jones's pro-choice views make him more attractive to national Democrats - his campaign already has met with representatives of Daily Kos, a liberal blog whose readers have donated millions to Democratic campaigns, including that of defeated Georgia congressional nominee Jon Ossoff. In Alabama, they limit his appeal to the rural conservative voters who once built the state's Democratic majority. Like Ossoff, Jones's campaign is designed to find moderate voters disgusted at the thought of being represented by Moore.
"I hear anecdotes from other Democrats who have Republican friends who whisper in their ear: 'I think I'm voting for Doug Jones,' " said JoAnn Cummings, a Democratic activist in deep-red Morgan County who drove an hour to see Biden and Jones. "They don't need to overwhelm us like they did with Ossoff. They just need to support the local parties."
Those local parties are now being asked to build a winning campaign from loyal black voters, liberal white voters and nervous Republicans. It's the sort of coalition that has elected Democrats in North Carolina and Virginia but never Alabama, whose largest urban county - Jefferson, home to Birmingham and some suburbs - has only recently become reliably blue.
The closest Democrats have to a playbook comes from Moore's 2012 comeback, when he was elected chief justice of the state Supreme Court but ran far behind his party's ticket, especially in cities and suburbs. While Mitt Romney won 60.6 percent of the statewide presidential vote, Moore took just 51.8 percent. Romney won 46.3 percent of the vote in Jefferson County; Moore scraped out just 37 percent.
That potential anti-Moore vote was visible in Homewood, the suburb of Birmingham where the defeated Strange still lives. The day of Biden's rally, diners at Salem's Diner - a crowded spot where Strange shot some of his campaign commercials - winced at mentions of Moore. Wayne Salem, the diner's owner, said that he had already decided to vote for Jones.
"We'd just be so embarrassed if Moore won," said Salem.
"It's awful, just awful," said Fred Blackmon, a 38-year-old stay-at-home dad. Like Salem, he marveled that Moore had been "removed from the Supreme Court twice," and worried that he would shatter the wall between church and state.
But if Jones turned out to be a liberal Democrat, Blackmon was less inclined to give him a protest vote than to stay home.
"If he moves to the middle, I could get more comfortable with him," said Blackmon. "I'm one of those people who votes with my wallet."