Harris, the US vice president, is expected to announce her selection by Tuesday, ahead of her first scheduled public appearance with her running mate that evening at Temple University in Philadelphia.

In a message to supporters late on Monday, Harris said she had yet to make her decision.

"I know many of you are eager to find out who I will be selecting to join me on the campaign trail, and hopefully in the White House, as my Vice President," she wrote. 

"Though I have not made my decision yet, it is important to me that grassroots supporters - like you - have direct updates about the state of the race," she said, offering voters a chance to sign up on a link to get news of the announcement first.

The choice of a running mate is one of the most consequential decisions of Harris' political career, as she hastily pulls together a campaign to challenge Republican presidential nominee Donald Trump and US Senator JD Vance, his vice presidential pick, in the Nov. 5 election. Vance will also make a campaign stop in South Philadelphia on Tuesday. 

Shapiro, 51, is a rising star of the party with strong approval ratings in Pennsylvania, whose 19 electoral votes makes it a must-win state for both Harris and Trump.

A former state attorney general, Shapiro would add to the ticket's historical significance; he would be the country's first Jewish vice president, while Harris is seeking to become the first Black and South Asian American woman elected US president.

Shapiro's strong support for Israel could alienate some progressive voters, though it could also appeal to moderate voters and defang Republican efforts to turn the Israel-Gaza war into a wedge issue for Democrats.

Walz, 60, is a former US Army National Guard member and a former teacher who has raised his profile in recent weeks as an effective advocate for Harris. He has attacked Trump and Vance as "weird," a viral insult the Harris campaign has embraced.

A former member of Congress from a Republican-leaning district, Walz has proven appeal to rural, white voters, though he has also championed progressive policies as governor, such as free school meals and expanded paid worker leave. While Minnesota is a solidly Democratic state, it is close to Wisconsin and Michigan, two crucial battlegrounds.

Speculation had focused on six finalists - four governors, a senator and a cabinet secretary in the Biden administration, all white men with a record of winning over rural, white or independent voters. 

In addition to Shapiro and Walz, contenders included US Senator Mark Kelly of Arizona, Transportation Secretary Pete Buttigieg, Kentucky Governor Andy Beshear and Illinois Governor J.B. Pritzker. 

The candidates will be informed on Monday night or Tuesday morning whether they were picked, sources told Reuters. The Harris campaign plans a social media announcement featuring the duo, campaign officials familiar with the arrangements said. 

Harris' search for a running mate began in earnest two weeks ago, shortly after President Joe Biden withdrew from the race and endorsed her to replace him.

There was no immediate indication that Monday's market selloff would have an impact on Harris' announcement timing. Trump, seeking to capitalize on the downturn, referred to the "Kamala crash" in a post on his social media site, Truth Social.

Over the weekend, Harris met with her vetting team, including former attorney general Eric Holder, whose law firm Covington & Burling LLP scrutinized the finances and background of potential running mates. Holder and his office made in-depth presentations on each of the finalists, according to multiple sources familiar with the process. 

Harris is weighing the decision with her husband, Doug Emhoff, brother-in-law Tony West and a small circle of aides and advisers, the sources said.