US vice president Kamala Harris will introduce her new running mate at a rally on Tuesday evening in Philadelphia — but with the event mere hours away, her choice remains a mystery.

In recent days, Ms Harris has zeroed in on a trio of potential finalists: Pennsylvania Governor Josh Shapiro, Arizona Senator Mark Kelly and Minnesota Governor Tim Walz, but decisions beyond that are not yet publicly known.

Her campaign planned to make the announcement via video message before the rally, though the exact timing remained unclear, according to a person involved in the planning who spoke on the condition of anonymity to detail discussions occurring behind closed doors.

Doing so would be similar to how then-candidate Joe Biden revealed Ms Harris as his vice presidential choice during the pandemic-marred presidential race of 2020.

It will mark another major milestone in the barely two weeks since the vice president moved to take over the top of the Democratic ticket heading into November’s election.

She has been scrambling to build out a campaign since then, and to breathe new life into the Democratic race against Republican Donald Trump.