In searching for his vice presidential running mate, Joe Biden above all wanted someone "sympatico" with his political strategy. How will he fare with his pick, Kamala Harris, who eviscerated him in debates and harbors her own presidential ambitions?
Chemistry is closely studied as a barometer of compatibility -- in governing style, policy, campaigning and political aspirations.
A presidential nominee and his or her partner on the ticket either have it and benefit, or they lack it at their peril.
And while there is visibly little to suggest Biden, 77, and Harris, 55, would be anything but agreeable political collaborators during -- and perhaps after -- their campaign to oust President Donald Trump, both nominees will be challenged to present a united front.
Mission accomplished, at least on Wednesday, when Biden and Harris appeared together for the first time as running mates.
Each had warm words for the other, with the former vice president stressing he has "no doubt" that he chose the right running mate and declaring to the Harris clan that "you're all honorary Bidens now."
Harris for her part highlighted Biden's "empathy" and sense of duty, and turned personal when she spoke of her good friendship with Biden's late son Beau, who served as Delaware's attorney general while she held the same post in California.
But in the era of coronavirus, the rollout event -- normally high-energy outdoor affairs with thousands of cheering attendees and loud music -- was a public-free zone: a high school gymnasium near Biden's home in Wilmington, Delaware, with almost no physical interaction.
While both nominees emerged together and delivered sharp, on-point speeches that targeted Trump and called for a rebuilding of a broken nation, there was little in the way of body language to assess.
Absent was the requisite hug or high five. No arm-in-arm double wave by two candidates locked in a delicate dance in which the running mate must appear competent and engaging while not outshining the principal.
Observers say that in his history-making two terms as the nation's first black president, Barack Obama's secret to his chummy relationship with his deputy Biden was that he felt little threat from the older Washington veteran.
In 2020 Biden is again the senior half of the political pairing, faced with a charismatic, confident and clearly ambitious partner from a younger generation.
- Team player -
Voters rarely care much about vice presidential candidates, but this year is different, argues David Barker, director of American University's Center for Congressional and Presidential Studies, because of the perception that Biden may only serve one term and is therefore grooming the future leader of the Democratic Party.
"There will be questions about whether she is a team player in the White House and not looking out for herself and ahead to the next election," Barker told a journalist roundtable.
Another "potential danger," he said, is that Harris is "so charismatic" and effective a political communicator that she could inadvertently highlight related shortcomings in Biden, who has been criticized as gaffe-prone and bland on the campaign trail.
Biden and Harris appear to share an affinity despite past clashes, including a searing exchange at the first Democratic debate last year when Harris assailed Biden's opposition to 1970s busing programs that desegregated schools.
Biden appeared taken aback by her assault, perhaps because he believed her relationship with Beau insulated him.
Trump himself picked up on their debate spat, saying on Tuesday that she was "very disrespectful" to Biden.
And the president's campaign has sought to paint Harris as radical extremist and powerful would-be "socialist" puppeteer pulling the strings on a Biden presidency.
If Biden sees himself as a bridge candidate that would help the nation transition to a new generation of leadership, as he has signaled, Harris may be considering herself the heir apparent, especially if the pair win in November.
But prior to the election Harris will likely focus on pushing back against Trump, elevating the Democratic platform and solidifying a bond with Biden.
"We're going to be seeing what level of vice presidential deference there is to Joe as the presidential candidate," Michael Steele, a former Republican National Committee chairman, told MSNBC.
"Does her star peak a little bit more than his?" he added. "All these dynamics are going to play out."