Former US. Vice President Joe Biden speaks during a rally in Philadelphia May 18, 2019. (Xinhua/Liu Jie)

US Democratic presidential candidate Joe Biden has promised to soon reveal his running mate.

And as the suspense builds over who Biden will choose to be on the ticket for November's election, this much is known - it will be a woman.

About a dozen names have been bandied about by pundits and the press but the favorite for the vice president slot appears to be California Senator Kamala Harris.

The speculation that the 55-year-old Harris would be the choice was fueled further last week when Biden appeared at an event clutching a notepad with her name on it.

Written on the pad under Harris's name were several complimentary remarks including "Talented," "Great help to campaign" and "Great respect for her."

The November 3 election is shaping up to be one of the most consequential in recent US history and comes amid a health emergency, an economic crisis and a national reckoning over racial injustice.

The choice of a running mate is always one of the most important decisions a White House hopeful makes but it has added significance in 2020 because of Biden's age.

Biden turns 78 years old on November 20 and he would be the oldest man ever to assume the office if he defeats US President Donald Trump in November.

Ronald Reagan was nearly 74 years old when he was inaugurated in 1985 for his second term in the White House.

"The vice-presidential pick this year is so much more important than it normally is because people expect Biden to only serve one term," said David Barker, a professor of government at American University. 

"And so whomever he picks as vice president is likely to be the next Democratic candidate for president in four years," Barker said.

Besides Harris, three other senators are believed to be under consideration - Tammy Duckworth of Illinois, Elizabeth Warren of Massachusetts and Tammy Baldwin of Wisconsin.

Two other lawmakers also appear to be in the running - Florida congresswoman Val Demings and California representative Karen Bass - and two governors - Michigan's Gretchen Whitmer and New Mexico's Michelle Lujan Grisham.

Atlanta Mayor Keisha Lance Bottoms has also been mentioned along with Susan Rice, who served as national security advisor under president Barack Obama.

Harris challenged Biden for the Democratic nomination but dropped out of the race in December 2019 and threw her support behind Biden in March.

AFP