Kamala Harris was set to announce her running mate on Tuesday before the pair head out on a five-day swing through America's hottest battleground states, just three months out from the presidential election.
The US vice president's campaign has come out of the blocks at a lightning pace since she replaced Joe Biden at the top of the Democratic ticket, smashing fundraising records and wiping out the leads built up by Republican rival Donald Trump.
Pennsylvania Governor Josh Shapiro, a Democratic rising star, is considered the favorite to join Harris as her vice-presidential pick, ahead of several other state governors, a US senator and Transportation Secretary Pete Buttigieg.
US media reported Monday that Harris had narrowed one of the most consequential choices of her political career to Shapiro and Minnesota Governor Tim Walz and would likely make her decision public in a video announcement.
Less than a day before their swing state journey, Harris was maintaining the suspense about her running mate, telling supporters in a text message Monday evening that "I have not made my decision yet."
Supporters hold signs during a rally in support for US Vice President Kamala Harris in Ambler. File/Reuters
Harris, 59, and her newly minted deputy will hold a rally Tuesday at Temple University in Philadelphia before hitting Wisconsin and Michigan on Wednesday, Arizona on Friday and Nevada on Saturday.
She was also intending to hold events in battlegrounds North Carolina on Thursday and Georgia on Friday, but local media outlets reported that a tropical storm battering southeastern states had forced their postponement. There was no response from the Harris campaign on a request for details.
Shapiro is enormously popular in Pennsylvania -- the largest of the six or seven swing states that have decided recent US elections -- and is seen as giving Harris a boost in what is considered must-win real estate for Democrats and Republicans alike.
The 51-year-old would be the country's first Jewish vice president, adding to the diversity of a ticket already boasting what would be the only woman ever sent to the Oval Office.
His support for Israel and handling of pro-Palestinian protests has sparked a leftist backlash, however, and Democrats are keen to ensure that their mid-August convention in Chicago isn't marred by progressive and anti-Israel activists.
Agence France-Presse