By Eric Bradner and Dan Merica, CNN

The two history-making figures who are linked together by Joe Biden are set to headline Wednesday's Democratic National Convention lineup.

Biden's running mate, California Sen. Kamala Harris, is set to be officially nominated as the Democratic vice presidential pick -- making her the first Black and South Asian woman to appear on a major political party's ticket and potentially the country's first female vice president.

And former President Barack Obama will make the case for his former vice president's election.

The speeches from Obama and Harris will be the most closely watched elements of a night that will also feature 2016 Democratic nominee Hillary Clinton and Massachusetts Sen. Elizabeth Warren, who was one of Biden's toughest 2020 Democratic primary rivals but in recent months has worked closely with his campaign on economic policy matters.

Here are four things to watch on the third night of the Democratic National Convention:

Prosecuting Harris' case against Trump

Harris, a former California attorney general known for her tough questioning of Trump nominees during Senate confirmation hearings, will, as Biden's running mate, prosecute her case against President Donald Trump.

Harris "hopes for people to see themselves in her speech," a Biden-Harris campaign aide said, as she will talk about not just her own personal story but also the experiences of others.

Harris, a daughter of Indian and Jamaican immigrants, has often spoken about both their experience in America and her own as a biracial woman. She's often drawn upon the lessons she learned from her mother as well as her time at the historically black college Howard University to explain her worldview.

The aide said Harris "will set out a vision for a more inclusive nation in which everyone is welcome and given equal opportunity and protection under the law."

She will also speak to the need to elect Biden, the aide said, showcasing her running mate as "uniquely the leader for this moment" while "drawing a clear contrast with the failed leadership of Donald Trump."

Rather than from her home state of California or Milwaukee, Wisconsin, where Democrats had planned to hold their convention, Harris' speech will be delivered from the same Wilmington, Delaware, venue where Biden will accept the Democratic nomination on Thursday night.

Harris, as other speakers have, is likely to tout Biden's character in a contrast with Trump. And she has a story to tell: She was close with Biden's son Beau, who died of brain cancer in 2015. The pair met when she was California attorney general and he was Delaware attorney general.

But she is also likely poised to play the traditional vice presidential role of attack dog -- a role she previewed last week in her first appearance with Biden in Delaware.

"He inherited the longest economic expansion in history from Barack Obama and Joe Biden. And then, like everything else he inherited, he ran it straight into the ground," she said of Trump then.

Obama to tout his one-time #2

Obama knows what it takes to be president. And he knows Biden.

That is why Democratic organizers view the former president's speech, more than any other on Wednesday night, as key to convincing voters that the former vice president is ready to jump to the top job.

Katie Hill, a spokeswoman for Obama, said the speech will outline "why Joe Biden and Kamala Harris possess the experience and character to lead us out of the ongoing economic and health care disasters that the current administration has blundered into."

Obama will talk about Biden's work as his vice president, especially his work on the economy and health care, and stress, Hill said, "the cynical moves by the current administration and the Republican Party to discourage Americans from voting."

"He'll make a pointed case that democracy itself is on the line -- along with the chance to create a better version of it," the spokeswoman said. "And finally, he'll echo his call to all Americans who believe in a more generous, more just nation: that this election is too important to sit out."

Obama made a fervent plea to elect Hillary Clinton in 2016, painting his former secretary of state as the rightful heir to his legacy. That vision didn't work out -- Trump was elected months after Obama's speech -- so the former president's speech on Wednesday night is an attempt to both rebuke his successor and boost yet another custodian to his legacy.

Biden's economic validator

Warren's warming relationship with Biden -- which led to serious consideration of a vice presidential nod -- is something of a surprise, given that she entered politics as a vehement critic of bankruptcy legislation Biden helped make law.

But their gaps have narrowed significantly in recent months, with Biden embracing Warren's policy proposals on bankruptcy reform, student loans, climate change that she'd already borrowed from Washington Gov. Jay Inslee and more. Those changes have been a product of consistent talks between Biden's campaign and Warren and her former aides.

The Massachusetts senator's cultural cachet with progressive women has already provided Biden a major fundraising boost -- including a $6 million haul during their first event together. But she might be even more important as a validator of Biden's economic plans, particularly with progressives.

Warren is set to speak Wednesday night. And while she'd likely have been allotted more airtime in a normal election cycle with massive in-person conventions, she showed during the Democratic primary that she is one of the party's most effective communicators of policy matters in short, digestible bites -- making her speech Wednesday night one to watch.

Clinton: Take nothing for granted

Clinton, the only other Democrat to face Trump in an election, will make a case on Wednesday that only she can personally make: Don't take anything for granted with this president.

Clinton, appearing from her living room, will say that the country "deserves a better president" and argue that person is Biden, a source familiar with the remarks tells CNN. But the overarching message in the speech will be that this election cannot be close and that Democrats must overwhelm Republicans in November because what Trump will do to win cannot be underestimated.

It's a message that Clinton is uniquely positioned to make. Democrats believed four years ago that the former secretary of state was on a path toward the presidency. Clinton lost her bid, crushing the party, and the former Democratic nominee will argue that this election is too important to let that happen again.

Clinton also plans to speak about Biden's and Harris' human sides. She will reflect on how she and Biden worked together during the Obama administration and reference how they have been there for each other when family members have died.

Clinton will also speak about Harris, too, with plans to discuss her "grit" as well as tout her compassion. She will mention Tyrone Gayle, a Democratic spokesman who worked for both Clinton and Harris before dying of colon cancer in 2018 at age 30. Clinton, the source said, will mention how Harris flew to New York to be with Gayle shortly before he died. Both the former secretary of state and the California senator eulogized Gayle in 2018.