As soon as Joe Biden announced his exit from the US presidential race and vice-president Kamala Harris pledged to replace him on the ticket, .
With just over three months until the November election, the candidate Harris campaigns alongside will be critical to beating in key swing states, experts told SBS News.
Biden's exit has scrambled Trump's campaign strategy of focusing on and gives the Democrats a great chance to recalibrate, said David Smith, associate professor at the United States Studies Centre at the University of Sydney.
"Democrats do have an opportunity to create a really fresh ticket which could motivate people who otherwise might've avoided the race altogether," he told SBS News.
In the 24 hours since Harris announced her intention to seek the nomination, several vice-presidential hopefuls have been quick to endorse her candidacy and rally behind her.
There are several frontrunners, each with various strengths and weaknesses.
Gretchen Whitmer
Gretchen Whitmer is the governor of Michigan, a key swing state for the election, offering 15 of the 270 electoral college votes needed to elect the president.
The 52-year-old Democratic star has strong national recognition and .
"One of the benefits of an all-female ticket could be that it moves the election right back onto reproductive rights, which is by far the Democrat's strongest issue in this election," Smith said.
Donald Trump won Gretchen Whitmer's state of Michigan by less than 11,000 votes in 2016, the narrowest margin of victory in the state's presidential election history. Source: Getty / Brandon Bell
Others like Morgan Rees, lecturer in policy and politics at Queensland University of Technology School of Justice, said Whitmer is unlikely to be chosen
Rees said a Harris-Whitmer ticket would be a "double barrier", as Harris already has to overcome "some major racial and socialistic obstacles just to get across alone".
Josh Shapiro
Rodrigo Praino, a professor of politics and policy at Flinders University, said he had put "most his confidence" into Josh Shapiro, the governor of Pennsylvania.
The 51-year-old former attorney-general and country commissioner is very popular in Pennsylvania, a swing state with 16 electoral college votes, said Praino, the director of the Jeff Bleich Centre for Democracy and Disruptive Technologies.
Vice-President Kamala Harris was in Pennsylvania with governor Josh Shapiro earlier this month. Source: Getty / Ryan Collerd
Smith said Shapiro also had appeal with , pointing to May polling by the Inquirer that showed more than a third of Republicans, 42 per cent, approved of his leadership.
However, Shapiro, who is Jewish, is also a staunch backer of Israel — an issue currently dividing Democratic voters.
Mark Kelly
As a former NASA astronaut, Mark Kelly has the potential to captivate the imagination of voters with his "compelling story", Smith said.
The 60-year-old senator from Arizona, a battleground state that holds 11 electoral college votes, is a former Navy officer who is respected for his work and scientific expertise.
Arizona governor Mark Kelly is a former NASA astronaut and could be a popular Democratic vice-presidential nominee. Source: Getty / Mike Coppola
Kelly has been an advocate for gun reform after his wife, former US representative Gabby Giffords, was left severely injured by a shooting.
Pete Buttigieg
Pete Buttigieg, the Biden administration's transport secretary, is also a possible pick.
At 42, the previous mayor of South Bend in Indiana is the youngest frontrunner candidate, which could appeal to centrist and Midwest voters.
The US Secretary of Transportation is 36 years younger than Republican candidate Donald Trump. Source: Getty / Kent Nishimura
"If he was the vice-presidential candidate, he would be the first openly gay man on a presidential ticket in the United States," Smith said.
"Perhaps the drawback with him is that in the past he hasn't done very well with minority voters, and certainly getting minority voters back is one of the real priorities for the Democratic party."
